Well what do you talk about on Instagram? Try talking about them with him in person. But if that's just too awkward, then try starting with a "hi how are you? I really liked what you posted or pinned or whatever that is you do on Instagram yesterday" (sorry, I don't have an Instagram). As you're talking and since you know him (sort of), then talk about something that he likes. It'll show that you're interested in him.
If you have any more questions, feel free to ask me.
