mentioned a chisel set, then later mentioned a scabbard chisel. I do not know anything about scabbard chisels. After looking at photos, I can guess.
Most of the basic bench chisel sets have four chisels - 1/4, 1/2, 3/4, and 1 inch. I asked my friends about it, and my friends and I use the 1/2 inch chisel more than all the others combined. Expanded sets typically add 1/8 and 3/8 inch chisels.
Welcome to the real world. These days, chisel sets labeled "1/4, 1/2, ..." may be true Imperial measurements, or they may be metric "equivalents". I have matching sets of Crown chisels with the same part number. The older, used set from a retired woodworker are true 1/4, 1/2, 3/4, and 1 inch. A new replacement set mailed directly from Crown was etched with 6, 12, 20, and 25 mm - the metric "equivalents" of my old set. (The reason they sent a replacement is a long story for another thread. Both sets are fine.)
Your nephew probably knows a lot about different kinds of steel. My opinion is that you will get a high quality steel from any bench chisel set from a recognizable brand. Any brand recommended by someone here will be fine.
When people ask me about choosing bench chisels, I tell them to look at the handle material and shape first. Don't worry about the subtle differences between steel alloys or socket vs. tang fittings; all are fine. Instead, hold the chisel handle in your hand in different ways, and imagine how they will feel to grip and use them. Are they comfortable? What if they were sawdusty or oily?
said "affordable." Consider picking up a few used chisels at the local swap meet. They practically give them away near me. Learn to sharpen and use them, figure out what you like, and then buy something that fits your needs later. Warning: Old chisels vary considerably in quality. Some will be superb, but some older steels are softer than modern ones. They will be easier to sharpen to a fine edge, but will not hold their edge as long. Great for learning!
POPULAR INEXPENSIVE CHISEL SET, HIGHLY RECOMMENDED BY SOME, BUT NOT BY ME:
If Pirate is looking for a true bargain new chisel set, consider the $6.99 WorkZone chisel set sold once or twice a year at Aldi grocery stores. You have to watch for the ads or hear by word-of-mouth. The famous Paul Sellers (a well-known hand tool woodworker) uses and highly recommends them. Some people here recommend them. I value their opinions, but respectfully disagree. I was not impressed with anything other than the $6.99 price for the entire set. I think they are awful. You get what you pay for. In case it matters, I bought them in early September 2018. I don't know if they appear on a repeating schedule.
Finally, if you are going to recommend a woodworking chisel set, you should advise your nephew that he should have a plan about he wants to flatten the backs and sharpen the new chisels. New chisels will not be sharp out of the box. Used ones will need sharpening. Whatever he buys will need a fresh edge from time to time, too. I do not know whether your nephew's knife sharpening skills can translate directly to chisels. A honing guide and a set of diamond stones might be a nice touch if you want to buy him a gift. Diamond stones are not cheap, but they last nearly forever.
Bonus - A nice gift for your nephew:
Make or buy him a wooden mallet, which is sometimes used for rougher work. He should not be using metal "knife-making" hammers on his woodworking chisels. ... unless he buys those Aldi ones. :-p