Tell him to man the fuck up and do it. If you can suck his dick, he can certainly lick your pussy. “Scared” is bullshit. He can google tips, you can talk him through it, etc. He has no reason other than his ‘grossed out’ thing. If he’s too grossed out by the thought of going down on you, then he can forget you going down on him! Quid pro quo!
Besides, you want to please your partners as much as you possibly can. That’s a natural instinct; to give as much as you receive. So for him to be so resistant to this is really when you need to start questioning who is putting who’s needs first? He’s being selfish.
I get being nervous about it, that’s normal, but he needs to push through the nerves and realize that it’s not scary, it’s not gross, and IT’S NOT ABOUT HIM. It’s about you. If he can’t understand that, then maybe you need to think about why you’re wasting your time with this guy.