Two different options come to mind,
First the Worksman The
Personal Activity Vehicle (PAV3). It looks comfortable and is not quite recumbent.
Personal Acitivity Vehicle PAV Tricycle from Worksman Cycles
The other is a conversion, if you can get him invested in it he will almost have to try it out.
three 3 wheel special needs bicycles
Just a couple of things I have run across that look promising.
Good luck
PS Be careful about letting him browse the Higley Metals site.