This might be on the expensive side but if you have a car you could buy him a bike rack or if you don't have a car then buy him a new bicycle for his birthday. You might be able to guilt him into cycling. I suppose if you bought him a rower and he hasn't used it then that might not work!
I'd say both DW & I are pretty motivated. I play tennis a few times a week and often have 4 hour matches on
a number of weekends. DW is a horse rider so she generally rides at least twice a week. We also go on a walk whenever we can, whether it's around the neighborhood or on the Chiltern Way, Ridgeway Path, etc.
I got DW a bike rack for our car so all I need now is my own bike. She got one about a year ago and enjoys cycling around the area and down the lanes.
I definitely agree with the idea that a complete lack of exercise and bad diet brings on sickness. I think my last sick day off work was over 5 years ago and I could probably count on one hand the number of sick days I've taken in my nearly 10 years of working. DW rarely gets sick as well even though she works in a hospital with lots of bugs around.
Another idea might be to get him one of those "Red Letter Days"/Adventure gifts for something like sailing or whatever. Maybe there's a sport out there he might like?
Not sure what else you could do besides hiding his car keys/bus pass?
Matt