How to approach a professor giving a presentation to work with him?
I would send an email a week or so before the visit, saying about what you have said here and asking if the two of you could have a conversation during the visit. That would be better, I think, that surprising him at the talk. It will give him a chance to review any opportunities that there might be. He might also think of others who might be able to help you. 

If you just surprise him with a request he won't have that opportunity to think about it, and the circumstances of the visit might make it impossible to make contact in an ad hoc manner. 

But if there is nothing he can do for you directly, perhaps the contact can develop into something longer term.