I'd say he's given you a pretty good signal that he's interested so next time you see him at the gym, just smile and introduce yourself. I can't imagine that he wouldn't, at the very least, shake your hand and introduce himself. After that, just make a comment like, "working hard I see", or "having a good work out?". If he doesn't engage you in at least a quick polite conversation about working out, then I'd just say nice to meet you and see you around. You don't want to be rude in the gym and interrupt somebodys workout. You never know if they do or don't appreciate that. If he doesn't seem to want to chat, let him finish his workout and say hey again next time you see him (unless of course you're in some kind of rush to get to know him). A few polite but short conversations can go a long way to paving the way for a more extended conversation over dinner.