Chaps might be nice. If he doesn't have sleeves, that might be a bang-up idea. I can see the leather apron also being useful.
I'd be careful with gloves, as the fit is really important, and you're going to be looking a different gloves for different processes.
Helmets? No offence intended, good lids start at a few hundred bucks. Me, I'd be happy to spend the money, as I've only got 1 set of eyes. Actually, hey.... if he doesn't wear prescription eyeglasses, get him a good fitting set of OSHA approved eyewear. That would require him to go with you, like gloves, you want a good fit.
Honestly, even though I know and understand that you want to do something really personal, getting him a gift card really has the benefits that he can get something that does fit.