I have two ideas. First would be a two person canoe with a seat placed just rearward of center for his solo use. This would give him adequate capacity and good seat position.

Second I believe there are two person kayaks that can have a seat removed and the other one slid forward for single person use. I have only seen that in passing and cannot give you brands or models. I think I saw them at EMS www.ems.com
If I remember where or find them I'll repost and give you the make and model.

jed