From what I see he could definitely use some muscle, especially on his hind end. A little bit of weight might help as well. Other than that he looks beautiful. It's hard to tell how his confo is from those pics but it's nothing horrible. He seems very high headed though, even for an Arab.
So all in all just do some work on flexing, collection and extension to get that head lowered, and plenty of movement and work for his shoulder and hind muscles.
Honestly, there are a lot of bitterly jealous people out there many of which we find ourselves boarding with. I have had it happen before to the point that they were abusing my horses behind my back. Eventually that one person got kicked out but it didn't stop the remarks. I wound up leaving that stable and going to a more laid back friendly place(unfortunately not everyone has that option). Just keep your chin up and worry about how you feel about him. Be proud of the horse you own. Best of luck with everything though! He's a beauty. :)
"Blame it on my wild heart."