I think they have little rubber claw-tip things you can put on cats that were developed as an alternative to declawing (I'm not sure, I could be making this up). Putting them on his front feet might not be a bad idea.

My sister's cat Booma ambushes ankles too, generally first thing in the morning. She's locked out of their room at night and, if they aren't having a mouse problem, has nothing to do all night. She takes it out on them in the morning but will generally calm down once she's fed.

Also, I'm sure Boo is a probably a typical shelter "mutt" but some cats (Maine Coons, for example) are known to be pretty kittenish until they're much older, sometimes 4 or 5 years. Who knows what sort of traits he inherited...