Hmmm....I'm guessing your son isn't a "gun guy" since you're the one posting the question here and not him....
In light of that, I'd definitely go with a low maintenance finish, either stainless steel or tennifer. I love Sig pistols, but they just rust too darn easily. The last thing he probably want's to do every day is scrape rust off his pistol. Sig now has a large lineup of stainless pistols, though.
I'd also get a 9mm, or maybe a .45. Both of these, I find, are much easier to shoot than higher energy rounds like .40 or .357 sig. In fact, I find the .40 very difficult to shoot in small platforms.
I'd go with something compact, since he's plainclothes. His role doesn't sound very active, since he didn't previously carry a gun, so I wouldn't worry about a bunch of backup mags. In fact, I probably wouldn't carry any. He'll have a SWAT team do any entries that he might need for search warrants, etc. and if he gets into a shooting, his best bet is to break contact. No vest, no radio.....the last thing he wants to do is shoot.
So basically, unless his employers are dramatically changing his role, he is basically carrying a gun to be an on duty CCW, for defense only.
I recommend a Glock 19 or maybe 26, and a SIG 239. Smith 3913s are awesome. I think the HKs are too bulky, but I don't have much experience with their smaller offerings.
Good luck.
-John
So rec