YES!!! There was someone near here that had an older horse that always looked thin.. she got visited a lot by the local SPCA but she had records to prove the vet has said the horse had an issue (age related) and he was not unhealthy.
One thing you could try with your boy is to add 1/4 c. Corn oil or canola oil to his grain ration to add calories. You will need to get some etra Vitamin A in him if you do this (mirra coat or something else like that) as the oil can interfere with the utilization of vitamin A. I have used corn oil to bring a horse up and add calories to the feed. It works well and will put a nice coat on them too.
I did not know about the oils interferring with Vitamin A. He's prone to sweet itch anyway, so the Vit A could be a really good thing for him.
He's 13.3H and no kidding, eats as much as my three Walking Horses combined
Big apologies to the OP for hi-Jacking your thread but, it's also great reading for you since you are concerned about your neighobor's horse