Here's an excellent source.
Clip-on.
Rio Mobility Attachable Handcycle - The Handcycles, Wheelchairs, Recumbents and more..Roll with us!
Chair style (rather than recumbent) handcycle
Top End Excelerator Stock Handcycle - The Handcycles, Wheelchairs, Recumbents and more..Roll with us!
These are only 7 or 8 speed units. Unless your brother lives in a flat area, he'll need something with more gears.
Can he use a recumbent handcycle? There's a much wider range of gearing available.