OP: The face-plant "myth" isn't as much false as it is undereducation. If you SUPER overinflate your BPW and think you can stay at the surface vertically, you're mistaken. You WILL be face-planting. However, if you inflate to just positive and lean back otter-style you're a LOT better off. It's absolutely relaxing once you get "on top of it" (pun VERY MUCH intended). As has been mentioned before, a BPW is the most flexible setup in terms of fit there is. For very small frames, you can even set up a "Switchback" style harness. I'm 6'6" 270#, and the same harness fits my tiny wife (albeit, with some slack and a little adjustment). If you get your grandson a decent BPW, it might be the last BCD he ever needs. Changing sizes is changing ~$15 worth of webbing and takes all of 45 minutes (to get it VERY close, a few dives to get it "right").
However, the BCD is neither a device to keep you afloat at the surface or a device to save money with. A BCD is to keep you neutral in the water. In the water, a BPW really shines. During dives, the BPW will keep him in a textbook horizontal position more easily than a jacket. The shape of the wing is VERY conducive to horizontal stability. If cost is a concern,
then this package is VERY hard to beat. It's low in price, and high in quality. If he doesn't like it, or doesn't like scuba, then selling it for a minimal loss is possible. That "minimal loss" is not possible with a "kids" jacket-style bcd. The other option is to get a super low-end and/or used kids BCD and deal with it. In the long run (next few years, I'm not talking decades) it'll be better to go with a BPW. It's what my kids will be in. Well, either that or sidemount gear