Are you rationing his water consumption? Lido drank like a camel when he was younger - whole bowl fulls at a time - then peed rivers, and my vet told me to give him frequent water breaks during the day, but not to let him drink a whole bowl at a time, and not to leave water out for free consumption. My vet said her lab did the same thing and unfortunately, never outgrew the habit, so she still has to ration water. Fortunately, Lido outgrew it at around 7-8 months old.
Are you using an enzyme cleaner like Nature's Miracle to clean up accidents? If not, the pup can smell the residue, even if you can't.
Good luck! I feel your pain. Until I started following my vet's advice, Lido was the same way at that age. Finally, the light bulb turned on around 14 weeks or so, and he was suddenly potty trained!