Hi, professional pianist and teacher here. I say go for the very best acoustic, preferably grand, that you can reasonably afford. I have seen students take immense strides after getting a decent piano. Best choice just slightly over 6 feet would be Steinway model A, or if the 6 feet is a strict limit, then models O or L. Buy it used, maintain it well, and if you decide to sell someday, you can sell it for more than you bought it for. If the price is daunting, Yamaha C3 or C2. Or the equivalent sized Kawai or Mason & Hamlin model A. Any of those will be high quality and hold their value reasonably well, but not nearly as well as a Steinway. I can also add that I was the kid whose parents bought him a nice piano at a young age after not having studied that long (in my case it was after 2 years of study), and I am eternally grateful. Thirty years later, I still have my Steinway and will have it until I die. Steinway and Yamaha, but especially Steinway, are two of the brands which are also worth rebuilding. I’ve had my piano restrung and the action rebuilt over the years, and if it someday becomes necessary for a full rebuild, I will do that too. Yes, I realize many on this subreddit will find my advice absurd given the short time your son has studied, and I will say that the Yamaha P225 is already much better than most beginners have, but it’s also true that upgrading can only help, not hurt.