I thought they became sexually mature at around the 2 year mark. But as to been interested I'm not sure. You'll have to buy him the playsnake magazine and see what happens.
Yeah, I think lads need to be around 500/600gm, especially if they're going to be working hard in a big breeding programme - otherwise they'll be ... er, will this get past the mods?... shagged out is my earthy term. (Well, it can do them serious harm to be bred smaller or younger but, like any adolescent male, they won't be lacking in interest.
Check out the Markus Jayne breeding ball python site - there is a tiny male pictured there, coiled up with his big lady. but he is way too small to breed imo.
And your girl needs to be in her third winter and at least 1500gm.