My Lexi is a few days shy of 4 months. When I brought her home I put her potty pad in a place where I could see it because I wanted to know when she went. I saw her every time she went potty. She got the Potty on the Potty Pad song and a treat each time.
I think you'll have to find a way to see him when he goes. Stalk him for a day or so
. After a couple of times he'll expect his treat and he'll stalk you until he gets it. You'll know the 'look' after a couple of times. Be prepared for a side effect, though. He may start to potty on the pad constantly just to get a treat. He'll probably even fake a potty by squatting on the pad then run to you for a treat. Lexi probably went a hundred times a day, most of them fake where the pad was dry as a bone! I gave her a treat anyway because it let me know she understood what was expected.
I had to change her treats one tiny piece of her regular puppy food because I was concerned about her getting too many treats. She didn't mind one bit. I pop that treat out, singing and celebrating and scoot it across the floor. She chases it sown like it's a hamburger and gobbles it up. Never mind that there a whole bowl full of it right next to her that she has absolutely no interest in!
I think consistency is the key and treating him every single time.