I have multiple HK 91 pattern rifles (all in .308) including 2 PTRs, and multiple AKs in 5.45x39, 7.62x39 and 5.56mm.
The PTR version your son is lusting after is a modified .308 pattern rifle adapted with proprietary components (receiver, bolt, etc.) to use standard AK mags and 7.62x39 ammo.
The HK pattern rifles are heavy, the ergonomics are weak with the charging handle and mounting of optics, and have mediocre triggers. But they have super cheap mags ($10-$12 for like new robust aluminum and only a touch more for steel) and widely available parts. It is also a classic 1960s battle rifle that is tough and reliable if the chambers are properly cut. Accuracy is 3-4moa for all of my .308s and if people tell you they are shooting sub-moa they are legendary magicians with rare rifles, or maybe have a PSG 91 sniper rifle that runs $15k. Doubtful.
AKs are getting pricey, but have widely available cheap parts, mags, and ammo. AKs are also easier to mount a red dot optic which should be considered. AKs are plain simple and fun and classic. AK ammo is around $200 for a case of 1,000 - way cheaper than .308.
I read of function problems with the PTR 32 years ago, but I'm sure they have the bugs worked out by now.
A good version of each rifle is now going to cost about the same now.
My opinion: the only reason to get an HK pattern roller lock rifle would be the advantages of widely available parts and cheap mags in .308, and the classic nature of the platform.
If I were buying a 7.62x39 ammo rifle, a good AK is all I would consider. I actually love AKs in 5.45x39 as the recoil is so light, but I loaded up on everything when it was widely available and cheap.
Edit: I forgot to say that the PTR will have pretty good adjustable front and rear sights compared to the AK, which are pretty crude by today's standards but adequate for their intended purpose.
