As a heavier rider myself, I feel comforted knowing I'm not alone.
I ride a 14.3 hand Mustang (John below), but he's built almost like a small draft horse. I LOVE his conformation because he is large boned and large hooved, but is still short enough for me to get on easily.
He seems to carry me fine, but I still worry about my weight and every extra piece of gear we carry (I trail ride him for 3-4 hours several days a week, in a western saddle). So I know all about the worry of how much weight is okay for him to carry. But honestly, he seems to do just fine so I try not to worry too much! And he gets breaks at the tops of hills and everytime we pass a nice pond he can get a drink and break if he wants it.
So have fun with your guy, you guys look great together!