Poor guy, he must have to really go.. well.. poo lol. Sometimes with sudden bloating like that happens right before they go.
If you have any Epsom Salt handy, go ahead and set him up with 1tsp per gallon, daily 100% water changes, fast him for the rest of the day and most of tomorrow. If he is still bloated then possibly wait til the day after. Keep him in Epsom for a day or two after the bloating is gone to make sure that whatever it was, is out of his system. Also try flaring him to see if he goes to the bathroom, as that may help make him go and it may take away the bloating.