I would stay away from neoprene if his skin is senstive. Neoprene doesn’t breath, it can rub if not fitted 100% correctly, and will make them sweat fairly easily. I use neoprene girths on some of my horses, and once in a blue moon we do get rubs. The one’s with more senstive bellies get fleece or cotton-web girths.
Here are some ideas to try:
Chafeless girths made by Wintec
Just plain ol’ leather girths with elastic on the buckles.
Also check out shoulder relief/anatomic girths as well!
Good luck finding something that works!