Don't get him a set. if you can, get him 3 DIFFERENT chisels from DIFFERENT makers. A 1/4", a 1/2" and a 1". One should be a Lie-Nielsen / Stanley Sweetheart. The second a Japanese chisel. The third a mid-grade or higher chisel from Lee Valley or Two Cherries or Sorby or Blue Spruce or Pfeil or ... For straight up comparison purposes without different blade sizes, the 1/2" is the best size to pick because it's likely the most used chisel for most woodworkers.
Why do it this way? Because the difference between a "good" chisel and a fanastic chisel is as much about how the woodworker and the chisel fit one another as it is about the steel and design of the chisel. Handle size and shape, as well as length and balance point of the chisel matter. Sometimes a LOT. Some people love/hate the physically smaller Lie-Nielsen/Sweethearts, others love/hate the feel of Japanese chisels, and some folks just wanna fill their hands with a big honkin Narex or Sorby. Understand that with new production chisels, there is a fairly good correlation between cost and quality, and as with many things, there's also a diminishing return. Once he settles down and has a good sense of what he wants in a chisel (other than "better"), then you (and others) can look at adding additional chisels over time.
In truth though, the best "investment" you can make in his woodworking at this point wouldn't be chisels. It would be sharpening. You could get the Aldi or Marples or Narex, or just ONE (1/2") high quality chisel. And get him some sharpening goodness. Again, a lot depends on how much you're willing to spend. You can get into sharpening for the cost of a square of plate glass, some wet/dry sandpaper, and an "eclipse" side clamp sharpening jig, all told, probably less than $30. Half hog, at least from my perspective, would be to get the
Lie-Nielsen Honing Guide and a pair of dual sided DMT diamond stones. Because the absolute core skill in hand tool woodworking is sharpening. The above is pricey, but will likely last a lifetime unlike sandpaper. Unlike waterstones, diamond stones aren't messy or fussy. Once he learns to sharpen chisels, he can sharpen plane blades as well. Encourage him to learn to sharpen knives. Boys and men LIKE sharp knives, and get serious cool points for being the guy who always has a sharp knife, especially when he sharpens it himself. (As long as they don't turn into sharpening geeks, but geekery over anything is always a risk with guys.) Once you have someone in the family who can keep the kitchen cutlery sharp, oh, happy days for no tomato will be safe. Avoid, at least for now, any of the power sharpening equipment. If he learns to sharpen well by hand, then he'll appreciate the power stuff much more, and also be able to put a better edge on things using a combination of power and hand.
Full Disclosure: I have chisels from 12 different manufacturers made in the last 20 years or less. I don't have any Japanese chisels. Yet.
Additional point: There are many varieties of chisels. What you want are BENCH chisels. Not ****. Not mortise. Not dovetail or skew or crankneck or paring.
Bench The chisel that does 90% + of chiselin'.
