Firstly, congratulations for responding so well to the girl so far.
Six months ago, my advice might have been slightly different to how it is now.
If you were living together, I would think that going from once a week to three times would be even more of a practical issue. However, because you live apart, I can see why he wants to fill his time over the other three days. Of course though, I absolutely see why this is hard for you. I live with my GF for half of the year and if she wanted to see a secondary three times a week, I would have a problem with it. But... we also have a child, a business together and she has a husband to balance, too. That would be a genuine cause for concern... but I would also use it as an excuse to justify my emotional discomfort. We can't do that.
I would have given you different advice 6 months ago, because I was still clinging to a primary model where I expected every decision to be made purely on the comfort of each other. (That's ok if it works for both people). I found that I was expected to adhere to this, but when it came to my GF wanting something, her needs were stronger than her need to please me. I could either keep ignoring this and telling myself that she was being unfair. Or I could say, sod it, just let go and let her make her own decisions. I can't tell you how much it has helped - and we haven't lost closeness because of it. From what she tells me, she feels more in love with me than ever. And I'm happier for letting go.
Your boyfriend is saying that he could balance two primaries. Of course he doesn't know if he can yet - he hasn't had the chance to explore that. That's not your fault - I would feel exactly the way you do. But... what if you looked at your relationship a different way? Force yourself to let go of possession and let him make his own journey, let him make his own decisions. I've realised restricting my girlfriend doesn't make her want to be with me more. Giving her freedom makes her love me more.
All of this being said, this doesn't mean you have to be a doormat. You should still be allowed to express your wants and desires. He feels that he might want two primaries, because he's never experienced that reality. You feel that you don't want him to, that he won't be able to balance it, because you haven't experienced him in that situation.
Ultimately, if your discomfort isn't enough to make him want to keep it as it is, you know that he is starting to need something more than he needs your comfort. Does that make sense? If he loves you, he won't be doing this because he's cruel. He'll be doing it because he cannot control his needs/wants.
You have to decide what is a deal breaker for you and what is uncomfortable. We use a number system for dealing with poly events. This is how our number system works:
0= completely comfortable, no issues
1,2 = normal poly pangs, minor
3 = enough cause to stop and think, quite uncomfortable
4 = seriously uncomfortable - compromising may help
5 = guideline broken or possible deal breaker for the relationship
This helps us, because it helps us to measure discomfort. "I'm uncomfortable" can mean a million different things. "I'm at a 4 about this" very clearly means "do it if you want to, but expect to do some work on our relationship if you want us to pull through". It can alert your partner to needing to give you extra time or care. It can alert ourselves to put a problem into perspective.
If I were in your shoes, based on how I feel, I'd say to my boyfriend "I'm a level 4-5 about this. But I want you to have what you want. I do not want to stand in your way. However, you should know that I am not sure I want to be in a full V relationship. Time will tell whether I am happy to share my primary partner on this level, or not. It might not work for me. But I will give it a 3/6/12 month trial."
I sense that your boyfriend telling you he appreciates your efforts, but he doesn't know how long they can wait is hurtful for you. I can understand that. It's incredibly difficult when you've already given so much of yourself to get to a certain point and then the goalposts move. It can make you feel unappreciated and resentful, regardless of his first statement. I've been there too. The only thing you can do in this situation is listen to what's being said and work out how you feel about it. Your boyfriend is telling you that he wants to move forward and doesn't think they can wait. He's made up his mind. That means you have no other choice but to accept it. If you put the brakes on, he will only come to resent you. He hasn't said to you "I'd really love to see her more, but if you're not comfortable, I absolutely understand and won't push the issue." I say let him know how uncomfortable it makes you, let him know that there might be consequences of unhappiness for you, that it might affect your future together, but that you want him to have the experience.
I do think you should speak to him about your concerns re: his current exhaustion. Don't go on and on at him, but state it clearly. Say that you are not sure he can balance and you are worried that even less of your needs will be met; but then follow up with "but I won't know until we find out". Because, in truth, you won't.
In terms of NRE taking sexual/affectionate time away from you... Certainly, my GF has had her head in her newest exploration. But I've been clear with her - I've said "You want a real life submissive, so have one. See him whenever you want. More than once a week is a level 4 for me. But do what you want to do. I'm worried that you will be so sexually caught up in him that our sex life will suffer. If that happens, I will not be happy in our relationship, as I need a primary who sees me as their primary sexual and emotional interest. I do not want to be a primary through emotion, but a secondary through sex. Those are my expectations and desires. Now, let's give it 6 months and see how it goes."
That's what I would do with your boyfriend. Yes, he probably will struggle with balancing at first. He's learning. Give him the opportunity to learn. If you do and it goes badly, it will go better the next time, because of the experience. Or, you'll realise that you need something else from a primary relationship. Either way, you come out of it knowing more than you currently do.