House Trained Dog Peeing and Pooping Inside?

My Jack Russell/Border Collie cross is 16 months old and was fully house trained up until 8 weeks ago. He has started pooping in the house again and it's not that he's not getting outside to go because we let him out nearly every 2 hours. Sometimes if it's wet it would be longer. He never poops while anyone is with him, but tonight for example I just let him out to go to the toilet then put him in his bed and I came to bed myself.

After 15 minutes I had to go downstairs for something and he had done four poops and had done a pee. This is happening most nights now and I don't know what to do as he gets to the toilet loads of times. I don't want to put him in the kennel out in my shed, but if this keeps up I'll have no choice. Any help would be brilliant.

By Rachael McC from Ireland