I love this part of your question, should he 'try it or not?'

I have to ask, why would he not? You said he's already a good programmer, has lots to add, he wants to be more involved, so I'd say, yes he should try it.

It isn't as if he is going to loose his programming skills. I actually think that depending on the structure of the project/company he is working with, a product manager who is also a great programmer can have a very positive effect on managing less experienced programmers.

If he finds he isn't very good at it, or doesn't like it, is there anything stopping him from going back to being a programmer?

When posed with a 'should I or shouldn't I' question, my default is always yes, unless I can come up with a good reason not to do it!