I realize this is out of your budget, but I'd get him the new 2017 iPad for $329
. Its been getting rave reviews and the price is actually quite good. iOS has a larger library of tablet optimized apps compared to Android and it has the whole 'it just works' factor going for it.
Your other option, if you're sticking to android, is the Nvidia Shield Tablet K1 for $199
. Its the defacto successor to the 2013 Nexus 7. CPU is plenty fast, it has a microSD card slot and front facing stereo speakers. Nvidia has, so far, offered very good software support for it. The only thing is that its mainly marketed and designed for gamers, but you can just ignore all the gaming features and use it as a normal tablet. Word of warning, this tablet does NOT ship with a USB charger. You'll have to buy that separately or use the one that came with his Galaxy A3.