Puppies, such wonderful little creatures aren't they? Keep you on your toes (to avoid stepping in puddles anyway).
Puppies have very short attention spans. You let them out they start to go potty then bam a leaf flies by they chase after that and forget the whole reason why they were out there in the first place.
I would recommend for a while to keep your pup on a lead while it is outside going potty. When he goes potty tell him how much of a good boy he is. He cannot come off of the leash nor can he play till his duty is done. Also, create only one area of the yard where he can relieve himself and take him to that spot every time.
It sometimes helps to keep a puppy on a longer lead in the home so that he is not out of your site and you can catch him the instant that he squats and at that point you yell a loud NO and take him directly outside even if it was only 10 minutes since his last trip outdoors. Have a routine with feedings, naps, playtime, etc. so that both you and your pup gets the idea. You will know when he has to go and hopefully he will go to the door to let you know soon.
Patience is key. you can do it.