Hi Zoomorphic,
Most guys are pretty gentle about it. If they're not, then they're definitely not worth your time anyways.
If he does reject you, just say something like - "ok, no problem, maybe some other time".
Ultimately though, if he does reject you, don't take it too personally. There could be lots of reasons for it that really have nothing to do with you. It could be he's already interested in, or seeing somebody else. Maybe he's just really busy right now and doesn't have time to go out. Could be anything really...
Look at it this way. If you never ask him out, you'll definitely never get a date with him. But if you do ask him, you might just get what you want
Go for it