Welcome to Modern Muzzleloading, Mom. And Merry Christmas. And a nice choice for your son.
Where is home exactly? states and cities can vary as far as stores and shops that handle black powder or in-line accessories and if we know where you're at others here who chime in may have expanded ideas for you.
Lets just keep it basic right now. First off, everything you look for should be marked 50 caliber. Check Walmarts or local gun shops. Here's a list of what I'd round up:
*Triple 7 ffg powder. Don't get pellets.
*An adjustable powder measure.
*bullet/sabot combos in the 250 to 300 grain range.
*209 primers....to get him started with the powder suggested you can use the primers packaged as "muzzle loader" primers unless you go to a gun shop that sells reloading supplies, then get 209 shotshell reloading primers. They'll both work with the T7 powder but if he ventures into BH209 powder later he'll definitely want the reloading primers.
* lots of cleaning patches
* a basic set of cleaning swab and brush
* a small bottle of dawn dish soap
* a small bottle of windex
* a package of sight-in targets
* gun case
This should get him off and running. You don't mention whether he has someone he hangs with that has gun experience or not but I would striongly urge you to take him to a gun club or a GOOD shooting store with his gun to have someone show him first hand some of the handling details specific to muzzleloading guns, especially the loading sequence. I'll also add here now that once the gun has a charge in it, the charge meaning powder and sabot/bullet, that the ram rod used to push the charge down the barrel be marked exactly where the muzzle ends on the rod....this is a visual reminder that he can see when he's loading the gun to be certain a load wasn't left in the gun from before or if he gets ahead of himself. This is called a witness mark and is the cheapest of all safety related things he can do for himself. A deep scratch in the rod's finish or a piece of masking or electrician's tape will suffice nicely for the mark.
As long as you are a registered member here now, I'll suggest that you encourage your son to sign in on your account and let him ask questions he may need to ask since he'll be the shooter. I'm not certain what the age requierement is for him to sign up and register here on his own, nor do I know his age so for now I'd say just follow your thread here with further questions. Really though there is nothing better than hands on learning about handling and loading of blackpowder guns. Like any gun they command respect and a full understanding that they can and do kill so having this instilled front and center right off is imperative. Honestly though you'll be hard pressed to find a better educational site than this one so you've made a super good choice with us here.
Again, WELCOME!