It would be a great pity if he had to give up cycling, it is sad that there are not more off-road cycle paths as there are outside the UK, separating cyclists and vehicles. Check with leisure activity or tourist information facilities covering where you live, and see what dedicated cycle routes exist. 'Sustrans' has developed a network all over the UK. using disused railway lines and canal paths etc. Some ideas; could someone take him out by car (bike rack, folding bike) to a suitable place. Maybe you could find someone to share this with. Everything is possible, there are even sainted people who take out the blind on the back of tandems!