Don't let him out - take him out. This is a really important distinction. It allows you to reward him once he has done an outdoor pee - and don't forget he may need a second one a few moments after, and needs to be rewarded for both - and it stops him being too anxious to get back indoors in case he is missing something, and so not emptying out fully. Don't fret him or say "come on" sort of phrases - just quietly stare into the middle distance while he potters about, then once he has finished his pee, reward him. Then, rather than head straight back indoors, let him potter about a bit more, just in case.