When you first play I would also recommend using safe-words, so that you can signal to him if he is reaching or overstepping your limits. That way, when you don't use them he will feel reassured that all is going well, even if you are crying out or in pain, he will know that you are OK. My girlfriend and I where discussing this the other day and agreed it would be fine as long as you don't have a memory like Dory from 'Finding Nemo' and forget the safewords!