DS7 started on pianp, then moved to the violin. Although he swears he loves it, the violin has been a tremendous challenge for him. On the piano, everything sounded ok even if not perfect. On the violin, there can be some truly offensive noise!
If the Cello is a little easier to keep a consistant sound on, I would lean that way, particularly if you have any issues with perfectionism. My DS is really little or I might recommend a switch.
I know people who have kids taking cello and I think either way, the set up for getting a good sound out of a violin or cello (or viola) is slow and difficult. My daughter is 6 and has been taking violin for a year and 1/2. I took violin growing up as well. I also have a pianist. Early piano is SO much smoother than early violin. Once the set up is there, it's fine, but the beginning can be provide some great (and difficult) lessons for perfectionists.
If he's more interested in cello, go for it!
