I can actually relate to that, though I have not had that problem in BDSM yet. There are some things that just make me laugh from sheer joy, and I cna see some people finding it offensive if one of those things is the way they act.

I would suggest sitting down and talking to him. Try to explain to him how laughter is sometimes not becaauise something is funny, but just because you are happy. Beyond happy. Estatic with joy.

Try to find something that makes him feel that way. Ask him what it might be. Help him experience the same joy you do. When he sees that laughter is a normal response to joy he will see that he is not being laughed at. This might take some time, and be a lot of work, but I am sure you can see that it will be worth the effort.