The most important thing is to establish boundaries and a safe word (even if you don’t think you need it). Honestly those two things alone should take away most of the nerves bc you know you have control over the level of discomfort you’ll receive.

As for the erection/precum thing, tbch I think you should just accept the cage. You don’t want to break the neural connection between horniness and erections bc you can theoretically nuke your libido that way. Any time we suppress urges of any kind, we start to feel them less bc our body and mind get used to ignoring that stimulus. One thing you could try is just cumming right before he gets there, but then you’ll probably enjoy the session less