If you want to start your kid off right, the first step is to purchase him some appropriate ANSI z87.1 rated goggles or a paintball mask. Don't cheap out on eye-protection, he's only got two eyes and they aren't worth losing over a game. In addition, good goggles or a mask won't fog up. Fog means he won't be able to see and if he can't see he can't play and won't have a good time.
Buy him some decent footwear - boots or something solid. Tennis shoes might be fine when the weather is dry, but it's no fun to have soggy, frozen feet.
Regardless of what AEG you buy, you're going to need to purchase a decent charger and decent batteries. The cheap wall chargers that some guns ship with are bad, will damage your batteries in the long run and should not be used. Invest in a good LiPo battery charger like the
Thunder AC6.
[I'm going to COPY PASTA my old, usual reply to these sort of questions below]
As far as AEGs go, tt's hard to go wrong selecting a gun provided you buy a reasonably priced AEG from a mainstream company.
For example,
Tokyo Marui
Classic Army
G&P
G&G
VFC
King Arms
Ares
KWA
ICS
Guns from the above companies should work well straight out of the box and shouldn't require any fixes.
Other "clone," "All China Made" (ACM) or low-price companies like
JG
Dboys
Cyma
A&K
make products that range vastly in quality and may often need a tune-up straight out of the box. I wouldn't recommend them for a first gun unless you're willing to do some work to get them skirmish worthy. I'm not saying these guns are terrible; they're not, but they are, generally, of lesser quality than the AEGs produced by the "mainstream" companies.
Now you'll probably get a lot of opinions about which companies are best/worst, which companies build the most reliable rifles, etc. That said, what you should do is settle on a particular rifle model (M4, M16, AK, G36, P90, etc) and then conduct some research to determine which company makes a reliable version of that model at a price point that you can afford.
For example, if you're interested in a G36, then you've got a variety of companies that you can look at including:
Tokyo Marui
Classic Army
Ares
JG
SRC
These companies produce G36s of varying quality at different price points and with different features.
So you've got to take some time and look at each company's G36 offering and determine which is best for you based on how much money you've got and other considerations like reliability.
In addition, as you delve into the gun buying process, you'll soon be overtaken by the urge to buy extra magazines and some sort of loading bearing kit or plate carrier.
RESIST this urge.
Remember, all you need to play is GOOD EYE PROTECTION, a gun, a high capacity magazine, batteries, batter charger and decent footwear.
Once you've got a gun, play a few games and figure out what works for you before making a big investment.
In my opinion, if you've got a $400 budget, it's far better to spend the majority of that money on a rifle, spare batteries and eye protection than dilute it in order to buy extra magazines and the equipment to carry it.
Good luck!