I just picked up two old Silvertones, similar to this model, only a little older, probably 1930's, no truss rods. This one looks like maybe late 1950's, early 60's with a truss rod....? They both need work (neck sets) and were purchased as projects with the intent to repair and restore. I only paid $165.00 each, so, although this one looks like it is in better shape overall, without being able to see the neck angle, it is difficult to determine whether or not it will be playable once the guitar is strung up and the bridge positioned correctly (you can see it has been bumped, and is sitting at an exaggerated angle).
Because these old archtops almost always need some repairs and adjustments, it is important to consider whether your Grandson will be able to make these repairs and adjustments on his own, with some guidance by others, or can afford to have the guitar repaired by a local Luthier. If the guitar needs a neck reset, this could easily cost the price they are currently asking. Although there are less expensive ways to perform neck angle adjustments (neck reset), sometimes the investment may just not be worth it.
Nevertheless, if the neck angle is good, $300.00 is still a little high and I'd offer something less, and negotiate from there
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