I covered this a little in a recent ask, and, unfortunately, sometimes sex will not be possible, no matter how much you prepare. However, discussing it and relevant boundaries/triggers in a comfortable setting (where the situation is not yet sexual) is an excellent first step, as a productive conversation can greatly help future intimacy.
A couple specific things that I have recommended before—the first is suggesting/offering that your boyfriend keep some articles of clothing on (such as his binder) to make him more comfortable during your intimate experiences if that would help divert/avoid particular triggers for him. Another idea is to set up a special code system to help you and your boyfriend communicate how he is feeling—having certain numbers, for example, to correspond to particular statements that may be hard for him to dictate could help you respond to triggers more quickly. Saying “I’m feeling dysphoric” or “that’s triggering”/”I’m uncomfortable” can sometimes be difficult when a person is having those experiences (and acknowledging them in such a way can sometimes make dysphoria worse), so a system of subtler signals is one way to make communication easier—and communication is key to helping the two of you feel comfortable.
More than anything, always try to be aware of his triggers; if you do not know what they are, ask his limits, and stick very carefully to them. Sometimes, he may be triggered during sex even if you are careful—this is ok, and no one is at fault. Try to be patient—though it can sometimes be frustrating for both of you, being extra careful and being willing to stop can avoid bad experiences that could negatively affect later intimacy.
If anyone has other suggestions, please feel free to submit!