How annoying for you! No advice really, other than wondering if he's got a dodgy tummy (as others have suggested). The only accidents Henry's had a couple of months ago were when he weeed inside because he had a slight bladder infection. If Alex doesn't normally do it it could well be a sign of an upset tummy. Or perhaps he's teething and feeling in pain? It could just be a minor blip - some dogs do relapse a bit.
I also remember some people saying that having the door open is bad for house-training, so perhaps you could leave the door shut, which might then remind him that he has to tell you when he wants to go out. And then when he does go when he's outside do the excessive praise again.
By the way, did you ever contact his breeder about the pedigree?