You're right, it can be tricky, but how close do you feel to him? If you refer to someone as 'my husband's son-in-law,' it kind of sounds like you don't know him at all. I dunno; I think I'd probably say stepson-in law and just say he's from an earlier marriage. I do hear people say this fairly frequently.

It's surely easier when you get past that and can use use his name! I will call him Steve.