Ask during work, but make sure that it's no longer than a few minutes.
Mention that you have a personal project where you're starting to use this technology, talk about it for a bit and ask him where he obtains resources to help out.
Don't explicitly ask for his help, let him volunteer that if he so wishes.
If he doesn't offer you some help, then don't press it.
Keep this in your head:
I will not be a help vampire, I will not be a help vampire
Of course, I'm not saying that you are a help vampire, but it's important not to start off appearing that you might turn into one...