I have a few thoughts here.
One, it is very hard for the average consumer to buy a diamond and know if they are getting a great stone or a dud because many diamonds will sparkle like mad under jewelry store lighting.
If your son is comfortable buying online there are other companies he may want to explore. My experience is that online vendors have pricing equivalent to OR BETTER in many cases than B&M jewelers.
Whiteflash.com is a company I have personally dealt with (but I have no affiliation to). My original engagement ring diamond was from 1996 and bought at a local jeweler. I replaced that with A Cut Above diamond from Whiteflash and the difference is remarkable. A Cut Above diamond are branded diamonds from Whiteflash that meet strict cutting criteria to achieve maximum brilliance and scintillation. Buying one of these diamonds, also called a super-ideal diamond, is like the EASY button of diamond buying. You are guaranteed a phenomenal stone. You can find a stone to fit most budgets based upon how you prioritize carat, clarity and color. There are other online vendors from whom you can get a super ideal diamond, some of which have their own branded versions - briangavindiamonds.com and distinctivegem.com are some other options.
Two, wherever he ends up buying a diamond, he should pay attention to upgrade policies. While not every couple would want to upgrade their diamond in the future it’s at least smart to know what your options are. If a B&M jeweler offers an upgrade policy it’s typically that you have to spend double the amount of your original purchase. Whiteflash allows you to upgrade for just one dollar more than your original purchase on A Cut Above diamond (also on diamonds that didn’t quite make the grade to be called A Cut Above but are really close - these are labelled as Expert Selection diamonds on their site).
Other companies, I believe James Allen falls into this camp, allow you to upgrade as long as you go higher in at least 2 of the “Cs” meaning you get a higher color and clarity or a higher carat weight and color, etc.
Three, has he considered buying a lab diamond? Lab diamonds are growing in popularity. Chemically they are identical to an earth-mined diamond. These are not CZs or other simulants. They are legit diamonds. But they cost a ton less than a mined diamond. I recently purchased 2 lab diamonds for earrings from Jonathan at distinctivegem.com and they are absolutely beautiful and cost me a fraction of what mined diamonds with the same specs would cost. There are multiple online vendors offering a great selection of lab diamonds - distinctivegem.com, adiamor.com, etc.
Fourth, this can be a very big purchase for a younger person. There is a forum at pricescope.com from which someone can glean a ton of knowledge about diamonds and how to pick a good one. The forum participants (mostly consumers but representatives of some diamond and setting vendors do participate although they aren’t allowed to make recommendations for obvious reasons) are very helpful and will even go so far as to help a new poster find a diamond by recommending diamonds that meet their criteria for budget, carat, clarity and color.
Fifht, note that most online vendors offer a discount (3% ish) if paying with a wire transfer. Check return policies but the ones I have dealt with also offer easy and free returns. So you can buy the diamond and look at it in the comfort of your own home in various lighting environments to decide if it is a keeper. Whiteflash will also create a video for you comparing a few diamonds if you are trying to decide between a higher and lower color for example. Other vendors may do the same. Jonathan at distinctivegem created a video of the lab diamonds I purchased from him showing me them in multiple lighting environments.
Sixth, if buying a diamond from a B&M jeweler it should come with a grading report from a reputable lab - AGS and GIA are the two most well-known labs for grading diamonds. Other labs may be softer on grading color and clarity so you might be paying for an H VS1 diamond but in actuality getting something more like a J SI1 diamond.