It IS a Yamaha, Japan...either a 21 or a 23. SO, NO point in buying a 23...because the Vito is that (almost zero differences between a 21 and a 23).
No, Vito stencils made by Yama never have a model # engraved on 'em.
It is a good horn, so IMHO unless you are ready to spend $800 for used or $1500 for new...which would provide you with a budget to really buy a 'step-up' from your Vito....there is little point in buying another Alto.
I would say, as already suggested....spend $250 or so having a tech put the current Vito into good regulation and removing any leaks. The fact that his current Vito is 'having problems' has nothing whatsoever to do with the model of horn itself, but rather with the current condition of the horn. A sax needs to be kept up, and then really every few years needs more than just a spot-repair/tune up once or twice. It may also be whoever you have been taking it to just isn't all that good a repair person on saxes.
Do THAT, then perhaps, if you really wanted to get your kid a holiday gift, buy him a mouthpiece or two....or a sax stand, or both.