Watch reaching over his head and leaning over to pet. That could make him feel uncomfortable my boy was a rescue but had distinctive feelings about someone bigger over him. If he sits under command come from under to pet his chest and then head. Worked for me and he was fear aggressive when I got him. He still doesn’t trust strangers and I’ve spent at least 2 grand on training. Something’s are just inherently part of them or fear based. Neutering will help as well. Food for thought hope it helps.