First off, be honest. do you actually find it unattractive?
He needs to trust you. Let him know that, even if it’s not ideal, you genuinely don’t give a toss.
Don’t let your sex life slow down. Even if things ease up for another reason, he might link it to his hair loss and start thinking he’s less attractive to you.
Shave that shit to the nubs, and blow him because he’s still a sexy sexy man.
I wish I had better advice, I don’t though. I have great hair growth, and baldness doesn’t run in the family at all. I’ve had a ponytail, and right now I ran a razor over it. I don’t understand people freaking out over hair loss. Nothing wrong with being bald.
If it’s a choice it doesn’t matter. If it happens to you young, fast and you have no control it’s really quite distressing. Balding is often associated with aging and oldness. When you are in your early 20’s it can feel like premature aging in your prime or having the remainder of your youth taken away. Plus if you are already very insecure as a person it’s really quite horrible.
You also probably look good bald man but if you didn’t you can grow it back 
Some of us cannot.
As a bloke with thinning hair, it was heartbreaking at first. But when my girlfriend(now my wife) gave me nothing but support and understanding, I found myself feeling far more confident and content with life.
Just let him know you still love him, bald or not.
The best way to support him is to accept his flaws. Set up an exercise date, a healthy food date, or a gym session together. If he really cares about you and you really love him, he’ll put in the effort. if i may add, being bald with beard is cool too!