I don’t know the answer now as to when I started this thread. However, a couple of related thoughts.
Not sure what son’s FIL would like, but if you go for heritage/history then either an Armalite or a Colt. Okay I know Armalite isn’t exactly the same company but the name would be at least a nod to the creator, as Colt would be to the first government supplier. Also think used in these categories. If the FIL might have some practical use, such as hunting, then also consider something other than 5.56. 300BO or .458SOCOM. If long range shooting, then one of the preferred 6 or 6.5 calibers.
If no special reason behind the purchase then as others have said, PSA might be the best beginners/tinkerers one to start. A gateway drug of ARs to speak.