Yep. He got the "germy" moodlet. I've worked out the cure though but thanks for answering.
I had built him a real shelter, walls, roof and all but I kept the floor as pasture grass. Originally, there was an open entrance, no doors just no walls. So, I closed up the hole, making it snug and warm inside. He started getting better and everytime I sent him out, he got worse. So to exercise him I opened the hole again and let him run out for only a few minutes. He's almost better now, I just have to keep him inside which is obviously a bit frustrating.
At least I know what to do!! Thanks for answering and is it possible to make this solved in anyway?