Stilt, I'm cold-natured myself, and like your son, I get cold easily and don't like it. If I were to stay above the 1st thermocline at Pelham, which is between 20 and 25 feet, even a 3mm is fine for me (you'll be above 78 degrees, usually in the 80s, the entire time). I wear the 5mm because I do drop below the 1st thermo (which we do here and there, depending on what part of the quarry we want to play in). It's always easier to flood the 5mm with water if I'm too warm, than try to warm up with a 3mm, if too cold. My 5mm is a Hyperstretch, which isn't even as thermally sound as a non super-stretch 5mm (but wow, it's comfortable!).
The only time I'd really find a 7mm necessary (and then some) is dropping below 70ft. or so. I've been down to the low 50's at 120 feet, at Pelham, and I simply don't go there unless there's a good reason (really not much to do at that depth there, other than just going to take a look, or do your AOW cert deep dive -- the viz does go up dramatically at around 100 to 110 feet, however, again, just not anything to see).
A hood also helps tremendously if you're cold natured (and if I know I'm going down to the bus, at 65 to 70 feet, which is actually just beyond the 2nd thermocline, I'll always wear my hood).
So, I find the 5mm a good all-around thickness for the quarries. It's much more comfortable than a 7mm (which can be a bit stiff and confining... at least for me).