You know, I do think that it's natural at times for long term relationships to start falling to pieces under pressure. Outside stressors, long-held resentments, poor communication, passive aggressive behaviour, and certainly unmet expectations. These things can sneak up without us even noticing.
The good news is that if you both see something worth salvaging, you can work together to achieve it. But it has to be together.
I have to say, it's not just you. On your end, it sounds like you could (and should) have communicated better and not been so rash as to break up. You clearly hit a rebound fling because you were hurt, lost and feeling destructive. I understand that this hurts your ex bf; but you were hurt in the first place. It's a chicken and egg situation.
From his side, at least as far as I can tell, he could have done some things differently too. Have each of you owned your mistakes and come up with solutions? Did you at all hint or try to talk to him before you broke up about his lack of attentiveness? Did he turn a blind eye?
Or was he genuinely a perfectly mature partner with perfect talking, listening *and* problem solving skills?
I have to be honest. Feeling rejected by your partner - feeling that they are not *present* in your relationship - can feel like a betrayal in itself. It's painful to feel that you aren't wanted. You didn't cheat on him. You didn't betray him. You were feeling out in the cold, with nowhere to go. Does he understand that part?
In terms of the improvements you are supposed to make over the next 3-6 months, are these measurable? Have you identified how you are going to communicate better? Have you talked about finding solutions? What about from his side, too?
You guys are facing a tricky situation now. I do understand why he thinks he needs options. However, I see a potential mess and heartache for someone... whether it's one, or both, of you two, or someone else who enters the equation.
In my opinion... you have around three options.
The first is to have a do-over on your relationship. Monogamously. Putting a time limit on it is fine and I do think this is a good idea. You could give it 6 months, with a review at the 3 month mark.
The second is the option that is being proposed. You stay mono, he gets to date others, until you've 'proven yourself' over the next 3-6 months. I really think that this is risky business. He's scared of being left holding the bag if you decide not to be with him, yet you're left holding the bag if he meets someone else. My worry is that if you both go into this with an imbalance in the power dynamic, that resentment will carry on. You're a team, not a criminal and keyholder. The ideas shouldn't be that you prove yourself, it should be that you do your part to communicate well and he does his. The other concern is that he could hurt someone else by getting involved with them while still being in love with you. That really isn't fair.
The third option is that you guys take an actual break and agree to see where life leads you in that time. When I've been stuck as to what to do with an ex, whether to continue, etc., those relationships have often ended on a 'break'. 6 months can make a big difference. You may find that life moves on and, actually, it's better than you thought it would be. On the other hand, you may find that you both can't stop thinking of each other and that with the space, you are in a better position to come back without resentment. However, this is different to him being poly and you being mono. This means an actual break, where you do not speak often, and you are both free to explore your other options; never having to talk about whether you've explored them, because you are free to do what you want to do.
Of course, you can go with the option that you wait for him to date around until he's made up his mind. You absolutely can do that. It is literally an option. As long as you are prepared for the chance that he might meet someone he likes and you might be waiting a while longer. However, as you say, if that is what he needs at this time, that is what he needs. You could choose to let him be; to let him go off and do his thing. But you two do need a clear agreement on whether this is a breakup, or whether this is a "let's review it in 3/6 months."