Even if the tone control is a homemade install, it's still a good deal. But just in case, be prepared to have these modifications undone, as people sometimes get them wrong and mess up the tone of the guitar.
But the seller is misinformed:
1) these can still be found occasionally (NOS) usually going for 1400eur.
2) the successor model APEX1 is still in production (list 2090eur).
Also, don't expect to easily resell it if your son finds the seven string is too difficult. I know people (professionals even) who just couldn't deal with sevens and gave up within weeks.
Lastly, even at 1,90m & >100kg, I found the K7 too heavy for my liking. The RG shape is quite thick and combined with the mahogany this make the K7 quite a beast to lift.
As others have said, the tone control is VERY
likely to be an aftermarket modification rather than "pre production" or "1st run" difference. If done correctly, there'll be no problems if it's done incorrectly you may need to have a guitar shop sort things out.
Don't worry about the seller's information on the market, one man's rare and out of production is another man's unwanted and discontinued. I'd have to agree with the seller though, I think these are becoming increasingly hard to find, regardless of whether the sucessor is still available. (Think PMC vs 7VWH
I'm a similar eight and weight to eviltwin and didn't have any problem with the weight of the guitar, but if the player is a slightly built 15 year old, it is something to consider.
Personally I'm a big fan of the guitar apart from the cosmetics, but I'm not a Korn fan. If yor son is a fan then this is very likely a great guitar for him. But ys, it will likely to be hard to sell on should his tastes change as there's apparently little market for these guitars and one that's likely to diminish further as Korn are no longer "flavour of the month"