Wow big boy. Well I know how frustrated potty training is I am trying to potty train both my kids right now. My daughter just turned 2 and my son is 3. He does better but he went backwards cause he saw his sissy in pullups and wouldn't go.
It has helped by training both now.
I know my son wears the 4t-5t in the training pants with Pampers and its the biggest.
The only one I know thats bigger is a brand thats actually degigned for Big kids to protect for bed wetting. There made like Pullups. Its called Pull Ups by Goodnites Underpants.
It says they are for Small-XL and for 38-125 lbs with leak guard to protect for over nites.
You can go to there website : http://www.goodnites.com
Hope this helps !