We have found our escaped gerbil! but need some advice! please help. full points!?

If you soften the food with a bit of water or milk it might help him digest it better, I'm not saying give him a full bowl right away.. but a bit more than that is okay. Also make sure he has fresh water. I'm sure he's pretty dehydrated by now. If the gerbils lived with each other before I'm sure 4 days isn't going to make a difference, I would just keep him in his own cage for now though so you can regulate how much he's eating/pooping/drinking, etc.