Holy ****, you don't come with easy questions, do you?
As a lover of our American classic brands, I'm sure there will be a lot of back and forth on this. Also, I've already begun my evening toking, so hopefully I don't just go on and on about nothing. I'm going to kind of address a few of your questions/points.
Fender Thinline vs Gibson SG - Umm... you don't even compare the two. They are so freaking different in just about every way. Tell your son he needs to play both and pick one. Go to Guitar Center and play a few thinlines and a few SG's. This is not a question anyone can answer for you. It is based on personal preference, and both are fantastic instruments.
On the two thinline Tele's you posted:
- Fender Vintera II - Made in Mexico
- American Vintage - Made in the USA
That's the reason for the price difference. However, the two guitars are also very different in other ways.
- Both seem to be made of Ash. However, weight of ash can range.
- The Vintera has a poly gloss finish while the AVRI has a nitro lacquer. The poly gloss is cheaper to apply whereas the nitro lacquer is closer to how they finished guitars back in the day. All custom shop guitars and most higher end guitars usually use some form of nitro finish. They tend to age over time and wear in, which many guitarists love. Others believe nitro guitars sound better. This is all just general discussions out there.
- The Vintera has the standard Tele bridge and neck pickup setup whereas the AVRI has the fender wide range humbuckers. I'm a Tele purists, so don't get me on my soapbox about putting humbuckers in a Tele. Suffice it to say, those two pickup options give you COMPLETELY different sounds. He needs to play a Tele with each type to know which he wants. If he wants a Tele because he wants to do that "Tele thing", then you need the classic setup with the single coils. If not the Vintera, then whichever other model. Granted, the single coil pickups in the Vintera are going to be of lower quality, due to the MIM. But those can always be replaced in the future.
Aside from those differences, I couldn't tell you without holding them. Both postings say "C" shape, but there are different types of "C" shaped necks. They are generally on the thinner side. Some people like chunky necks, some like thin necks, and many of us like necks somewhere in between. That's all a personal choice.
The posting also shows the Vintera has the more old school 7.25" fretboard radius vs the 9.5" of the AVRI. Again, that's personal preference. I will say, the more more extreme the radius, the more likely the guitar is to fret out when you bend a string. The likelihood of fretting out could come down to the quality control of the guitar itself. I personally love the 9.5" radius Fender uses for their re-issues, these days. But most guitars range from 7.25"-12" radiuses. Everyone has their own preference and they all feel VERY different.
If you can post the serial number of his current Fender, I could look it up and get the specs on that. Based on the specs of his current guitar, you could then ask him what he likes about it and what things he would prefer different. For example, if he loves the neck shape and has spent the last 3-4 years getting very used to it, then let's try to get a similar neck shape in the new guitar.
As for the Gibson SG's, COMPLETELY different style of guitar and sounds completely different. Those are both Gibson SG Standards, so both are made in the USA and basically have all the same specs.
If it were me, I would buy the Pelham Blue one, if he wants an SG. First of all, that's a premium color to get in an SG. Second, it comes with the Gibson hardcase. Third, that one comes with the Classic '57 pickups, which are MUCH better than the trashy 490R and 490T pickups in the other one. I think those are the worst Gibson manufactured pickups and would rip them out the moment I bought that guitar. The Classic '57s are much more vintage voiced for classic rock.
But, my recommendation is to first make him make the choice between a thinline Tele and an SG. If he chooses the Thinline, come back and post the serial number of his guitar. If he chooses the SG, snag that Pelham blue one.