It really depends on how much of the crest is fat and how much is actual tissue like tendons and muscle. Horses can get a cresty neck for whatever reason; breeding, conformation, being cut late, being obese, etc. It probably couldn't hurt but I'm not sure how much it would really help.
A diet would help your horse's neck. He could be IR or early stage Cushings. I'd look in to a low sugar, low starch diet with an added IR supplement. You should check out safergrass.org and pets.groups.yahoo.com/group/EquineCushings for diet recommendations. I'd also have your vet pull blood and test him for IR, Cushings, and any other metabolic disorders.
A cresty neck on an Arabian (or whatever light breed he is) is definitely not normal.