All Skylines are VERY sturdy and reliable cars. They're also quite cheap to repair as there an entire industry servicing them with 2nd hand parts.
Servicing costs are on par with any other six cyliner car.
Fuel consumption is also typical small 6 clyinder, although, repairs will typically be cheaper than a similar FRONT wheel drive car.
So, all in all, they are not a bad option, HOWEVER, I strongly recommend a manual gearbox version. Your son is at the beginning of his driving career, and if he doesn't master a manual gearbox now, he likely never will. It's my humble opinion that if a person cannot manage a manual, then they do not deserve a license. he has his manual license, and in my opinion, moving away from a manual now is unwise.
A Skyline of this vintage is an excellent "learner" vehicle for manual transmission drivers, because they really are virtually bulletproof.
*** Be aware that he needs to be expected to get pulled over by the police a lot more than if he drove, say, a Lancer. Especially if he's out and about at night on weekends. So he'll need to behave.