He's got about an acre of grass- some flat, some hilly.
You have gotten a lot of advice. What is right for your son (and you) depends on what your expectations are. There is a reason there is there is a 10X increase in price from the cheapest to the most expensive "mowers". While they all mow their capabilities, the way they feel, and how long they will last are all are very different. Ultimately, it comes down to how much are you willing to spend and what are you will to put up with?
Deere makes a bunch of different models that are a couple of hundred to a thousand or so dollars apart. They do this so there is always something better/bigger/easier to use/more powerful/more capable you can purchase. Depending on your property a 100 series might be more than adequate and you might even like it. Most people who fit into that category never visit a tractor forum. They have a solution that is working for them, why would they waste their time there?
What you find places like here are people that have had a bad experience (for whatever reason) with a machine and what to talk about it or learn more to keep from having that again. 15 years ago I bought an X300 after I test drove it. At the time I knew nothing about mowers or tractors and thought the 100 series was what I wanted. The dealer got me to try the X300 and I loved the way the Kawasaki engine sounded. It was supposed to be for my wife (I am allergic to grass, so did not plan on using it, though I was looking longingly at the bigger machines). A few years later I learned about the "issues" with the K46. My X300 came with a K46, so I got worried. For a decade I worried about the K46, but it was fine and I don't have a flat property. A couple of years ago I upgraded to an X534 because it has diff-lock and I was tired of having to extract my mower every once or twice a year that I got it stuck. I like the X534 a LOT better, but it doesn't really mow ANY better. However, it doesn't get stuck in places the X300 used to and I can take it places I would not have dreamed of doing with the X300. Now, I paid more for a 15 year old machine than I did for my X300 when it was new. If you are thinking 100 series price, the price of a new X500 series is going to be unreasonable. I felt that way a couple of years ago, which is why I bought a used one that appeared to be in good shape. But, that has its own challenges, as it took me a year to find one, pre-Covid.
The S100 to S200 series all share similar frames and most major components. They do have different engine choices. Most people think Kawasaki makes better engines than B&S, but they all generally work reasonably well if maintained well. They do sound different.
When you get to the X300 series you get the better frame, but the transaxle is still going to be a K46 or perhaps a little better. The better ones, also have the ability to change the oil, which should allow the transaxle to last a lot longer in most cases.
You have to step up to the X500 series to get a really good transaxle. I think this is worth the thousands of dollars extra it costs, but mostly for the diff-lock. Since I have gotten more than a decade of service from my X300, I can't say the K46 is bad. I do understand the concern and this is why I have 2.5 spare transaxles waiting for my K46 to die. It might also explain why mine has never died 😉.
The X700 series is in a completely different class. I have an X749, which some consider to be the ultimate one, diesel, AWD, AWS. I don't think it is a good mower for my property (too big, too heavy, too tippy). But, my property seems to be quite extreme. I have the back 0.40 that has some fairly significant slope. It is mostly greater than 10° with about 1/3 that is at about 15-18°. Since there is a fence that goes across the slope, I have to mow part of it side slope. This is very dangerous. I didn't really realize how dangerous until I measured it and read the manual. It is also why I added bar tires and had them filled. It is also the reason I got the X534. It REALLY handles the hills well. If you don't have a challenging property, you don't need such a capable machine for just mowing. Remember that I used the X300 as my mower for more than a decade and it was fine most of the time. I didn't really like using it, but it was fine. I love driving my X534 everywhere on my property. I still like my X300, it just isn't as much fun to drive. My property is not nice suburban lawn. Much of it is trails and pasture that is lumpy and bumpy. I am pretty sure that would have wrecked the front-end on the S100 or S200 series machines in less than 5 years. That probably would have been a strong incentive to upgrade machines much sooner. The more expensive/better machines will last probably as long as you will want to keep them, as long as you are reasonable with maintenance. I expect my X534 will be okay well into its twenties and maybe past 30. Most 100 or 200 machines will be mostly used up in a decade or less. The choice is yours.