Before you follow him, think about the reason why you want to do that. Once it becomes clear, it should make you answer that question better. One thing's for sure- if you like him, you definitely should.
WHAT A GUY THINKS WHEN A GIRL RANDOMLY FOLLOWS HIM
Guys mostly get puzzled whenever a girl randomly follows them on Instagram. Especially for cute guys who are a little oblivious about their own appeal or attractiveness.
Some guys are very confident about their profiles, as well as their looks. They just probably want you to take a look and be amazed. However, he could also have a genuine reason.
If he hasn't followed you back yet, he's probably not that interested. The sooner you accept it, the better your life will be in the long run. Maybe he's just not that into you.
It could also be because he's been really busy. Maybe he's a really busy guy who knows his priorities. He could be super into you, it's just that he doesn't have the time to flirt back with you.