The “drop” is the difference between the shoulders and the waist, in case a salesperson throws that word at you. Meaning if he is 42 jacket, most suits will have a 6 inch drop, so the waist of the paints are 42–6=36. the most you can take in a trouser at the waist is 2 inches. There are suits with 7 inch drops, but they are tough to find, especially at moderate stores. Most suits carried by the average retailer have jackets made for average to fat men, so if he has a trim waist, the suit jacket might be large across the mid section. It can be taken in a little.
Calvin Klein sells suit separates, and can be a great option for young men, and you can get the jacket and pant in the exact size you need without alterations, except the hem on the pant. I’m pretty sure they still sell the separates at Macy’s, although I haven’t been shopping for men’s tailored clothing there in a few years.
Do not buy a jacket that is tight under the arm or tight around the upper arm.
You said he needs a suit, and then said he wants a blazer and slacks, so which is it?
Edit: just noticed I read that wrong, you said he doesn’t need a suit. Ok, here is my advice, buy him what he would use at most wedding he might attend in the future.