I had piano lessons from around 8 right up until I left School at almost 17, I also had oboe lessons from about 12 to leaving School too (in many ways I was the same as your son with this, I didn't practice it nearly as much as I should but was probably better at the oboe than the piano even although I didn't particuarly like the oboe, played in the orchestra too).
Probably depends what your son intends doing, I was all set to go to University to study music, had thoughts of teaching music, and at the last minute took a massive u-turn left School and became a secretary.
Does your son's School offer any options to hire a cello?
I have in the last few months taken up playing the piano again and have bought a Yamaha Clavinova, which apparently are approved by the Royal School of Music. Older Clavinovas are often available for sale on e-Bay.
I understand though that it would be very frustrating to buy these instruments for him just to give up.