so you know.
compressed air and nitrogen tanks are the same things, co2 is what you want to stay away from.
Also, what kind of gun does your kid shoot? you may need a specific type of air tank. Some guns run low pressure (less than 150 psi) while some require higher numbers 400 and up. You need to be sure the tank you get has a proper regulator AND/OR is adjustible to the range needed for his gun.
If this is supposed to be a suprise, get me the name and model of the gun and I can tell you what type of tank you need.