How old is your son? I assume he's a teenager or older because he's already maintaining a pretty large tank himself. Me, personally, I think a lot of the joy in this hobby is setting up the tank and purchasing all the things yourself. Weighing out all the options and seeing the different tank ideas is all part of the fun. If you're set on getting him a tank, an ADA one would be top of the line -- but at least if it were me I would rather get a gift card for such a thing so he can enjoy picking out the tank and doing all that himself.
$300 is a really good amount -- for a mini tank you should be able to get almost everything for that price including the tank, substrate, lighting, and some for plants. CO2 (depending what you get) would be a hundred or over so that might be a little extra.