Anyone can get heat rashes, diabetic or not.
Higher sugars may not help him get rid of it but I wouldn't have thought would've caused it, the first time I got a heat rash was when I was on holiday and whilst my sugars levels weren't as good as they are now, they were a lot better than when they had previously been consistently high (which was most of my childhood and teens and twenties lol) when i never got a heat rash
When I had said heat rash anti-histamines helped
Probably best to get it checked out by the docs.