This.You're a good friend to come here and try to find ways to help him. It speaks well of you.
For starters, just let him know that you care about him and will not desert him... and remind him often. ENFPs can be sponges for affection, affirmation and reassurance. Keep inviting him to do stuff with you - whether a hike or cup of coffee or over to your place to watch some TV. Even if he turns you down frequently, just being invited and included will make him feel supported.
It's a bit of a depressed ENFP tendency to go "I am annoying to be around, so I'm not going to contant anyone for weeks. --> No one is contacting me! I guess I'm unlovable. --> I am more depressed now. I will especially not contact people."
As a friend, anything you can do to throw a wrench in that cycle is helpful.