Be more exiting. play with him, toss toys.
never chase a puppy or you will ingrain a habit of running away from you that will become a huge problem. If puppy has something he shouldn't use a high pitch happy voice and run in the opposite direction. If you can have a toy to play with or high valu treat (during the training phase you want to have treats ready to go at anytime). reward when he comes to trade/play.
now if you are out strictly because you need him to potty and not play, then consider doing so on leash (so potty breaks are on leash) and train to go on command.