I agree with ginger, his prostate could have something to do with it. Is he neutered?
Ghilly
When he does his poop, how does that look? Normal, or does it come out kind of 'squashed-looking' as if it has been flattened?
Reason I'm asking is his poop might look like that if he has a swollen Prostate gland. If he has, it could be obstructing the flow of urine slightly.
You could ask the vet to check his Prostate. This might be a slightly uncomfortable procedure for him, but not painful.
Dogs, like men, can get prostate enlargement as they get older.