I'm sure you've probably already thought of this, but just in case you haven't...a padded toilet seat might help with the sensory issues/cold seat problem. You can usually pick one up at Walmart or Home Depot for less than $20, which is way more economical than keeping him in diapers. Everyone else in the house will probably appreciate it, too.

I know this is only one piece of the puzzle, but every obstacle you can remove makes it a little easier.

Good luck.