Hi nonamenoname,
I would just hang around him. And be consistent in how you contact him and how often. Like, I used to text or make a phone call once a week and hang out once a month for a certain friend of mine. That’s just an example. People like patterns. When he knows you’re often (not always, don’t smother him!) there and dependable, that will eventually make him feel closer to you. Like a brother, probably.
Keep it simple, don’t over analyze the friendship, and relax!
Inky