Same way you do in a monoship. Keep it at friends, do not tell him about the crush, do not flirt. You can enjoy the crush feelings from a distance.
You sound like you assume that if/when the time comes that you are ready to date him, he will be at a place where he is wanting to date you back. For all you know he will transfer again. There's lots of potential Right Ones out there. They don't always come at the Right Time. So best to just enjoy this one in the shape he came in -- a coworker friend type that you crush on from a distance.
That's the least
of your probs.
Bigger is DH.
When you Opened, you initially agreed to those limits. You are finding some of the limits no longer fit in practice and will grow harder to stick to as time goes on.
It is fair to alert DH of this development, and try to renegotiate those limits. You have been honest. You have not cheated. It is fair to ask to revise an agreement that no longer fits the needs of all parties so that it can be updated to meet all needs fairly again. The happy medium.
Is this NOT how how he wants to be treated? With fairness, compassion and ethics?
Would he rather set you up to wither inside because your needs are unmet and you live knowing his need to avoid discomfort for himself is greater than him meeting his responsibilities of seeing to his partner's (you) continued well being?
Would he rather set you up to be tempted to get them met elsewhere because he refuses
to hear you or talk to you? Effectively shutting you out and dinging your emotional intimacy together?
This is way more serious than the crush dude.
If DH is overwhelmed and flooding, it's ok for him to ask for a time out to regroup. But don't skip the talk forever -- a time out has an end point. What does he think he needs? A week? A month? A reasonable time frame please, so he gets his time out to regroup. But you get assured he isn't going to play the avoidy/foot dragging thing FOREVER on answering these:
He doesn't think it "right" for you to be having such conversations with someone you would date down the road.
He does not elaborate or explain WHY he does not think it is "right." What is wrong about it? What is his fear speaking to there?
He does not give you a chance to make agreements for how you will date men at all -- so how can you magically know "what is the right way" then? Will you have opportunity to develop those agreements with him? When?
Or do you need to be coming to terms with the fact that his limit of you not dating guys is not a SOFT limit that could change in time. It is a HARD LIMIT that will never change.
Ask him to please be clear with you. So you know where you stand. Then you can decide for yourself with full information whether or not you want to stay in relationship with him under this "no dating men" agreement or not.
If he habitually shuts down communication with you and does not give you the right to responsiveness? Eventually you will have to move on to decide if you want to stay in relationship with a partner that shuts you out like that. You cannot exist in a communication vacuum.