I can't speak from any personal experience, but it seems that a few of KD's coasters can accommodate people of a larger size. It's all about proportion to be completely honest. I'm not sure about Dominator's (B&M ride) larger seat if it is the same as Alpengeist or Griffon's. Intimidator 305 is very forgiving. You just need to be able to get the seat belt buckled. Drop Tower is the same way I believe. If he can fit on Nessie, then he should be able to fit on Anaconda, though Anaconda has seat belts which could cause some trouble
WindSeeker (in some cases), the wooden rides, Flight of Fear, Backlot Stunt Coaster, and Shockwave don't seem to be that forgiving.
Most rides have a test seat except for a few (WindSeeker or Ricochet come to mind.).
There are a few flat rides that you can try out such as: Beserker, Bad Apple (formerly Triple Spin), Triple Spin, Flying Ace (mini-WindSeeker as some may call it), Scrambler, and a few others.
Hope that helps!