You can purchase jumpsuits that zip up the back, making it impossible for him to undress without assistance. Use your favorite search engine to look for "adaptive clothing." Two such sites are: www.silverts.com and www.buckandbuck.com.
If you sew, you can make some yourself by altering some of his current clothes: Double-stitch pants and top together at the waist, stitch closed the front-plackets and pockets. Then finally sew a long zipper into the back, from the middle of the seat up to the neck. The end result will look like a regular outfit, but will be "tamperproof."