He wants a monster truck? Monster truck he shall get! Bonus...the whole "kit" thing is the right way to go about building something IMO. I'm all in for kits. Putting in the motor you want; the ESC you want; the servo you want; the battery you want, and the radio you want...that just plain awesome! Today's problem is RTR RCs out number any monster truck kits. I think the Pro-MT would be a great platform to build (probably about the only one ). Just make sure parts are available. Also, if 4wd is something on the wanted list, check out the Tekno MT410. Then again, that one is quite a bit more money, and doesn't even include wheels, tires, or body .

Battery, motor, and ESC? 4000mAh ~ 5000mAh 2S LiPo battery would be plenty of capacity (runtime). My general rule of thumb for "C" rating on a pack when it comes to bigger tires needing turned...nothing less than 40C (A Guide to Understanding LiPo Batteries ). For a ESC/brushless motor combo, I'd have a look at something short course-ish in specs. A 3650 sized (36mmx50mm) 3800Kv motor controlled by a 120A ESC would be a suggestion from me. The quality of motors varies from manufacturers, but a 4 pole 3650/540 sized motor should do the job in a monster truck (if not going bonkers on the Kv rating). If wanting somewhat of a "sure-thing" in a motor, turning some huge knobby tires, then you probably would want a little longer motor (3656/550 size). HobbyWing EZRun WP SC8 3656 sized motor combo, or EZRun Max10 SCT 3656 sized motor combo deserves to be looked at. A WP SC8 ESC, by itself, can be had for around $54 (USA) if you shop around.

Lots of possibilities out there to put in the chassis. What gearing you have, and how fast you want to go, will be your biggest dilemma.

Sounds like you and your son are about to have a great time.
Good luck.
Get out there and throw some dirt!