Check with your pharmacist or GP, but I think you can put a touch of emulsifying appointment on his chin to help with the rash. It's very similar to vaseline. Some parents don't like to use ointments like this though, which I appreciate. Just check before hand but I'm so sure I used this! Also, when he dribbles, don't dry it with a muslin. It seemed to make the rash worse for my daughter as muslins can be quite rough. I'd dab it dry with a cotton bib.
We never suffered that badly with dribbling but more with pain - I was a convert to amber neckaces to reduce the pain but also Cheeky Monkey cheek rub which you rub on the outside of their cheeks to help soothe and calm... not sure it would prevent the dribbling or the rash though?
Ooh, I was going to suggest the dribble bibs too :)
I've heard @nipple cream can be good!
Dribble bibs to stop the wet tops however vasaline worked well on both my two's chins :)