It's not too unusual for dogs to have lapses. Gracie has regressed a couple of times. I started giving her treats every time she did it right and it seemed to jog her memory really fast!
Is it carpeting he is going on or a throw rug? Many throw rugs have a backing on them that have a scent similar to the smell of urine to a pet. They can cause some issues. He could be drinking more with the hot weather and he may not be able to make it all the way to the pad on time. I could think of a bunch of other reasons but treats are usually the best answer. (Unless he has a UTI)