What should I write to him first?

A first message gives the other person something to respond to. One real detail from a profile or a shared conversation can make it specific, but an elaborate opening cannot ensure an answer. Curiosity works best when it is actually yours.

If there is no detail you want to ask about, a plain greeting and invitation is complete. The response can show whether a conversation develops; you do not need to invent a shared interest or a personal story to begin.