That usually means he's going into shed (convenient actually - they often skip meals while shedding anyway). Otherwise a pink belly can be from a burn from an unregulated or improperly regulated uth.
He'll be ok, if he's a healthy weight he can afford to go a few weeks without eating. Once you get rid of the mites he can go back to his normal business. The mites likely came either on him or on his bedding or decor, if it came from a pet shop. Generally mites are easy to avoid if you have only one snake, don't bring other snakes into your house, and don't visit other smaller very often. Of you go to a pet store or reptile expo and handle any animals you should shower and change clothes immediately when you get home. But snake mites can only come from other snakes.