He may have been overfed before you got him and once they are mature adults it's hard to lose the weight. If he wasn't pooping regularly I would wonder if he was impacted from the wood chips possibly. Is he very active at all ? A couple things you ccann do is feed him insects only 2 times a week and reduce the amount but greens are fine every day. Let him out of the tank to run around in a safe room, be SURE there are no loose objects on the floor that a beardie will swallow like buttons, coins, rubber bands , tiny doll shoes, lego or game pieces. Also be sure he can't get under the fridge or any holes in a wall, etc. Some exercise and reduced insect feedings may help him to lose weight.