Need advice

So bondage has always turned me on. I've been with my husband for 11 years now and I know all of his fetishes. He love feet and peeing. I like those too but I'm really into bondage but I don't know how to tell him. I always do things for him. Wear what he likes and stuff but he's got really bad knees so we mostly do one position... Spooning. I mean it's ok sex but since he can't kneel I don't know how to approach the subject. There are times where he is pain free and those nights are amazing. We also have 4 kids so he doesn't like quickies or anything and I'm worried he's going to use that as an excuse not to make bondage a part of our sex life.... Any ideas?