He is still very young, so I doubt he really is potty trained this point. How often are you taking him out? He can probably only hold it for 3 hours tops, during the day I would be taking him out every hour, besides after eating/playing/etc.
Also, always put him in his crate if you can't watch him.you can also tether him to you so he can't sneak away and potty.
When they are this young, accidents in the house are human error, so don't punish him.