Everything mimi said, and don't give up. My partner has had dogs in the past and they took a very long time to learn. Longest was 6 month. He was a slow learner in everything.

My discussion with her regarding your dog:

Take him out ON A LEASH every two hours or so.

Thing to realize is that HE IS GETTING DISTRACTED WITH PLAY and forgets why he is out there. If he gets distracted with play, bring him back in crate him for 5 minutes then take him out again. Repeat until he does his business.

And lastly, you may just have a slow learner. Just like people some catches on really fast, some take time.