There are SO many variances in conditions, but, FWIW:
I have the condition to the point of needing a cane to save a potential fall when walking anywhere. Really, I don't have trouble cycling, with a few exceptions. I need to visually check my foot position regularly if riding without toe clips, otherwise knee trouble starts within a short time. He'll need to keep those joints lined up to save possible injury. My left leg has actually developed better muscle bulk and tone because I can't press as hard with the right leg.
Handily, I don't need a cane when out cycling; using the bike and brakes for support when walking works out fine for me. If I have to leave the bike, I've got trouble! Getting on/off can be risky, but again, lock the brake and it should help. A step through or mixte may work out without the performance downgrade a trike would entail.
Good luck to both of you; you may be pleasantly surprized at how much he's able to do, I hope so anyway!
Last edited by North Coast Joe; 08-17-13 at 06:19 AM.