The Sigs I have seen would be pretty large to be comfortably worn in an ankle holster. And a pistol that large would be hard to conceal in an ankle holster and would be very obvious to anyone who looked. When I used an ankle holster years ago, it carried a little S&W Chief"s Special .38 revolver.
On the other hand, the Sig Sauer pistols I have fired were all very high quality guns, and really nice. I like the .40 cal, as it has quite a bit better stopping power than the 9mm.
Is your son a Cop? In any case, he should do his homework on the local concealed carry laws to make sure he is in compliance. Although most likely the fact that he is packing would never become known to anyone, in some areas having a concealed or for that matter unconcealed firearm will get a person in a whole lot of trouble if they come in contact with law enforcement. I mean guns drawn, down on the ground commands and maybe even being booked into jail.
I sometimes used the ankle holstered revolver as my off-duty gun, and tried it as a backup gun when I was in uniform. But I found that having a holstered revolver on my ankle made it tough for me to run very far, which occasionally I had to do on patrol. So my backup weapon was usually in a pocket or an inside the waistband holster.
In retirement, I seldom carry a weapon anymore. But I also don"t go some places where I think I might possibly NEED a weapon. So far, so good.
Good luck to you and to your son!