The Star Lite also rocks, though I will readily admit that it (pretty much only obtainable new) is rather expensive. As cool as it is, I would have to have a long sitdown with it and the Cerveny 653 and 654.
Assuming he already plays Bb and Eb, F would be an easy pickup. If he plays Bb F would make a better contrast. And, in contrast to what I just wrote
, I think the 653 can be a very decent only tuba if he wants to play only 1 horn. I've played the 653/5 and play a slightly smaller Eb every single day. The 653 has a big fat tone, and extrapolating that against my smaller Eb's success in concert band and especially brass quintet and small ensemble I think it'd be a fine choice.I wouldn't pick a small bass tuba if I were the only tuba in a large concert band, but I have done just that (played, not picked to play
). Maybe not a 6/4 sound, but it is SOOO much fun. Bob likes being unleashed. 
In the same price range, an older Besson/... 19" bell 3+1 comp Eb or one of the clones might be a nice choice if one desires a bigger bass tuba.
