They probably aren't related. A small percentage of people are allergic to glycerin, and some people have reactions to certain artificial sweeteners. Though, if you're allergic to glycerin, VG might not be your main problem. Glycerin / glycerol is in a LOT of stuff, you might want to take him to the doctor and see if he's allergic to it.
This is the same thing I wonder when people say they are allergic to PG. How do they brush their teeth, use deodorant, use soap or shampoo, eat ice cream? (I would not want to stand next to them on a hot day

(Retired Nurse. I understand the difference between "allergic" and "reaction"
He's not allergic to anything else with VG in it. He's not allergic to VG at all.
He has a bad reaction to warming and INHALING it. (And I don't think it was ever studied for inhalation like PG has been)
I can't abide the non existent TH of VG.
So in answer to the poll, I choose PG.