I saw the photos (and to me he looked almost liver chestnut, so it’s like “the white/blue dress thing” all over again). Honestly, some well-conditioned horses (of all breeds) are always going to look a little thin to the eye and others (unfortunately) even if they’re underweight, are going to look stocky, because of their build in photos.
According to the OP, the vet was concerned and gave a BCS of 3. I’m not saying all vets are always right, but I’m going to assume the vet did more than look at the horse in a photo, and probably that has more significance than our impressions from a still image.
To me, he looked very under-muscled and weak in the neck and a little weedy (not racehorse-lean), so it would make sense if he was underworked and a little too thin before 12 hours in the trailer he looked less-great after that. But that’s just from the photo, so who knows if I’m right?
Again, I don’t see any harm saying to the owner what the vet said, in case he drops more weight, despite quality feed. I certainly wouldn’t send him right back (even for humane reasons) or expect a discount based on what I personally saw, because stuff can happen during trailering, but if the OP buys, and the horse is still not putting on weight as he should, then it’s something I would bring up with the purchase price, and make it clear what the OP started out with.
Hopefully, a few weeks of very light work and lots of food and TLC will make a big difference.