When he's in school to get certified he's gonna get huge discounts at the "tool truck" companies. Mac, Matco, Snap on, Cornewell, etc. These are considered some of the best tools.
Before that, there are options to get him nice stuff. Personally, I really like the craftsman kits that come in a big plastic case. Decent tools, good storage system, and if/when he upgrades to nicer stuff the case is useful.
Depending on what he has already that's a great way to knock out a ton of tools, it'll have minimum a ratchet in 3 sizes, a ton of common sockets, and wrenches in common sizes.
I store some of the trays from that case in my tool box, 2 of them fit in one drawer sideways. When I get better stuff I plan to put the case back together and use it as my truck tools, same as I did before I got the gig I have (and before I was working out of a box in a shop)
Without knowing what kind of tools he already has it's gonna be hard to make a specific recommendation.
FWIW I'm kind of a mechanic. Not certified, not great by any means, but I work on stuff for the company I work for. My setup is based around a craftsman kit my parents bought me in highschool and I think it's one of the best things I ever got. It really gave me the stuff I needed to feel like I could build something and I rebuilt my first engine with mostly that kit.
Other things I added that your son might need/want: Missing sockets. Maybe he needs deeps, or 6 points (all the 1/2 stuff in my kit was 12, imo very little uses for 12 unless you're tightening ARP bolts) or he needs specialties. Good specialties are torx and allen sockets, long versions, etc.
Lots of extensions are good. IMO you can never have enough. Go grab a LONG 3/8 off ebay for him. You can get a used 2 footer from craftsman or even truck brands for under 20 bucks shipped. Don't always need it but when you do it's awesome.
Swivels are a good tool. Also called universal joints. They have a knuckle that allow you to get in tight spaces.
Hammers hammers hammers. Very important. Go get trusty cook, there might be a discount code somewhere on this site I think. look for posts by "hammersmadeinusa" They make hammers for a LOT of companies, even some of the biggies. They sell some packs that include useful hammers.
Prybars are useful.
Socket driver is a good stocking stuffer. basically an extension with a handle, IMO sometimes it's better than using a ratchet.
Pliers, cutters, clamps, brake hangers, caliper compressors, vice grips, tape measures, screw drivers, etc.
Tool storage is good but if he's going to school to get certified he's gonna get great deals on really good stuff. Simple. He can always keep anything he has now as his "home" kit or truck kit. Most of the mechanics I know either have a bunch of cheaper tools at home for minor stuff or they are constantly taking their tools from work.