I have been a Master Pistol Instructor and Master Rifle Instructor as well as a certified Armorer for numerous firearms manufacturers for over 20 years. I also testify as an expert witness in both state and federal court regarding firearms evidence and I'm in my 27th year as a police officer in a major jurisdiction. Just to give you some info so you can decide if my advice is worth your time.
Choosing a duty weapon is not a decision you can make by figuring out what is a "reliable" or "desirable" firearm or what someone else likes or prefers. Kudos to his department for allowing him to choose his sidearm!!! I feel that is extremely important!
Please ask your son the following:
1. Why doesn't he like the Glock .40 (I assume it's a Model 22)? The specifics of his answer is how you will search for the new sidearm.
2. Has anyone ever helped him check his natural "grip angle"? Everyone has a natural tendency to "point in" . UNLOAD THE GLOCK. Then check to make sure it's unloaded. Then check AGAIN to make sure it's unloaded. Have him hold the pistol in front of him in a two-hand grip in the "hunt" position (depressed muzzle position). Tell him to pick a point on the wall at about eye level like a picture or wall light. Tell him to close his eyes and visualize the point on the wall. Have him quickly raise the pistol to a firing position WHILE KEEPING HIS EYES CLOSED. Freeze. Have him open his eyes and determine where his sight picture is in relation to the point on the wall. Is the front sight way below the rear sight so his shot would go really low? Is the front sight way above the rear sight so his shot would go really high? Are his front and rear sights nearly level? Different pistols have different grip angles. Some of us naturally point in at a grip angle that matches the Glock grip angle. Some of us naturally point in at an angle that matches a more upright grip angle such as a 1911. Can he learn to shoot a pistol that does not match his natural inclination for grip angle? Of course he can, but it may be easier (and more comfortable) for him to find a pistol that matches his natural "point in".
3. Is it the recoil/muzzle flip of the Glock that he doesn't care for? He should try a 9mm Glock pistol. Don't listen to the pop culture gun "experts" that bad mouth the 9mm. The 9mm round in its different variants has killed more people all over the world than any other pistol round by about a multiple of 50. I carry a Glock 17 (9mm). No one I work with wants me to shoot at them so it must be a decent round. All of our MFI's (Master Firearms Instructors) are trained to watch our shooters that have smaller hands or less grip/wrist/forearm strength for the tell-tale sign that the .40S&W round is not for them. You can see that shooters with smaller hands/weaker grips will absorb the recoil from the .40 and the gun will actually twist slightly in their hands. Glocks have a right-hand twist to their polygonal rifling and a .40 round is actually hot enough to translate to the recoil causing the gun to twist as the muzzle rises if the shooter isn't strong. In my experience it's actually more likely for a "less strong" shooter to be more comfortable with a .45ACP than a .40S&W. The problem with the .45 becomes the size (circumference) of the grip. If it is the recoil/muzzle flip that your son doesn't like, warn him that the polymer-framed pistols are much more forgiving than steel or aluminum-framed pistols. Super slow-motion video shows that polymer-framed pistols bend and flex with every shot which translates to less felt recoil.
4. Is it the trigger that he doesn't like? That's a no-brainer. He need to try some different pistols until he finds one that he likes.
Most importantly, don't look for a "bargain". There are cheap guns, there are good guns that are excellent purchases for homeowners and people with concealed carry licenses, and then there are firearms that can take the abuse of a daily duty weapon. Keep your manufacturer shopping list short, perhaps focused only on those pistol manufacturers that have built guns to compete for military contracts, etc.
I'm happy to help in any way that I can. Feel free to PM me also if you wish.