My 15 month old basenji boy has not had an accident in the house for many, many months. He was slow in housebreaking as he came from an elderly breeder who had him and his 4 sisters locked in a single crate 24/7. He ate and peed in the crate for 4 months until we rescued him. We have a 2 year old basneji girl who has no house breaking issues! The past week, our boy has peed and/or pooped in the house almost every night! WHY? We take them out every 4 hours or sooner … they sunbathe in the backyard for hours a day. Any ideas as to why he could have taken a step backwards? Thanks for you thoughts!
My Housebreken Dog is Peeing In the HOUSE!
i dont think this helps - but this was my experience. Hope was easy to toilet train right from 9 wks she never went in the house. and then there was a period were she would pee in the house again (unsure of the age frame) but we were so confused as to why she went from being so perfect to being a little brat we had to watch her again and then after a month or so she came good again.
shes not desexed so it might have been her marking or when she got cranky with us she would punish us by peeing on the bed or carpet (little sh!t). it was like she was going through a rebellius stage. Of course we got her checked out by the vet to make sure she didnt have any UTI's or other issues as sometimes they can do this to let you know "hey, there something up. this is the only way i can tell you".
anyways - thats my experience. i am sure many others will have answer(s) for you. maybe worth getting him checked just to be sure everythings well? 
Firstly I'd advise you to get him checked out by the vet in case it's a medical problem.
Did he experience any changes or trauma just before he started to break his housetraining? Something may have triggrered his reversion to his earlier days.
I usually advise to go back to basics with his house training to sort the problem and then it's amatter of patience.
I agree with Patty. A vet check on abnormal behavior is always best. I had a Basenji who never touched anything in the house at 14 yrs old chew up a chair cushion. A vet checked confirmed a bad UTI. He actually pee'd blood waiting in the vet. My fault for not taking him in right away when I suspected something.