Our Chow 'Beau' just turned 10 months old, the groomer we brought him to bath and groom thinned his hair and main. This is my third Chow so I know that you never cut a Chows coat. My questions is will his main and coat grow back to normal.
Oh goodness, so sorry to hear that he was cut and not shaped or trimmed. When I worked at a vet we had to shave chows to clean and prep areas on the body for surgery (a MUST in those cases unfortunately). His coat will grow back, it just may end up being a little patchy. Give it time, like 4-6 months, time... Chow-chows are double coated like many dogs but their undercoat is much thicker, so as his coat comes back in it will look messy. Keep up with your daily grooming routine, brushing/fluffing. Know that he will be even more prone to matting now since chows' undercoats come in first, before the guard coat does. De-Tangle at the first sign of a matt starting to clump. After the coat has grown back fully, a very experienced groomer might be able to lift under and lightly use thinning shears on his undercoat in some of the patchy areas, to even him out.
Add Omega Fatty acids to his diet to encourage a silky coat, there is no food or supplement that will speed up growth, just help with shine, strength, and overall skin health, and reduce breakage. Try salmon oil or missing link skin and coat.
You're going to have to have patience, but his coat will grow back in time. I hope you got your money back from the groomer (just my opinion).