Lots of hugs your way! It does sound like a difficult situation.
My DD often sleeps for 13 o3 14 hours at night (with the extra couple of hours in the morning), so it is not impossible. Especially if she had a night where she woke up more than once or twice.
In the daytime, at his age, she would sleep 2x 2-3 hours . So some babies just seem to need more sleep than others. But, when I took the luxury and had a nap with her, she'd sleep for up to 3.5 hours. By herself, she'd go for 2 hours max, and even that meant resettling her once. So could you try lying down with him every now and then (might be good for you, too).

Some babies are easier aroused than others. So I wonder whether his short naps have anything to do with more noise around him (birds, cars, you washing dishes, etc.) and more light.

Oh, and he surely is one big boy!

Sasa