Even with the low price of mostly decent AR15s, it's still cheaper to buy your own. I haven't built one from commercial prices in a while but last I checked it was cheapest, and easiest, to buy a complete barreled upper and the rest of the parts loose. List:
Complete barreled upper receiver
Handguard (unless it is included with the complete upper)
Bolt Carrier Group (unless it is included with the complete upper)
Lower receiver
Lower Parts Kit (usually comes with Fire Control Group, can get without)
Buttstock (unless it comes with the LPK)
Receiver extension and hardware (unless it comes with the LPK)
Grip
Sights &/or optic
There's a good "AR15 assembly guide" on AR15.com (imagine that) which is easily googled. It's about the only redeeming value that website has. Good guide with good documentation. You don't necessarily NEED all the tools and gadgets they show, but some are handy. For instance, you don't /need/ to buy the 'puck' to set the lower receiver on, but for installing the pins into the trigger guard, it's vitally important that you support the receiver 'ear' lest it snap off. The puck makes that easy, but there are myriad ways to accomplish that with items you have on-hand.
Otherwise the tools required are minimal and basic. Pin punches (ideally a set of roll pin punches, too) and a small hammer. Painters tape to mask areas near pins when installing. A spanner wrench for the receiver extension castle nut. Easy to make, or it's like $9, or maybe your son has one.
I think that's about it, off the top of my head. Once that's done, just magazines and ammo.
I do it to save money and to get exactly what I want, but I'm picky. I buy cheap parts for the components that don't matter terribly much, and buy more premium stuff where it counts, and end up with a great performing rifle for the cost of a cheap off-the-shelf rifle. You could do a lot worse than a S&W M&P15.
Good/cheap sources for parts and stuff:
Brownells
MidwayUSA
Palmetto State Armory
Bravo Company USA
I don't think I've ever really bought from anywhere else unless it was an availability issue or a crazy sale price somewhere else.