You curb your jealousy by realizing that he is just your friend - nothing more / nothing less. If you want him as a boyfriend, then you need to talk to him about this. Otherwise, you should never be jealous of anyone's friends.
You're not competing for anything. If you want him to do something with you and he's busy, then you find something else to do with your time. Your social calendar should never be ruled by one person. If he's your only friend, then that's something you need to work on. It's not his mission to keep you happy and busy. That's on you.
The sooner you realize this, the better your life will be. Be happy and productive on your own. You want your friendship to enrich someone's life, not be a noose around their neck.