My dog won't go outside or go potty he just sits?

My dog clyde I've had him about a yr and he's an older dog idk for sure how old but he's usually really good about goin outside to go potty and he loves going out doors he'll wait by the door all day till I let him out and recently he just changed completely he will take a few steps then just sits down and won't move for nothing I'll stand him up and he will jus sit right back down he won't go potty he will just sit and if he walks before he sits he drags his butt a tiny bit. He's usually obedient he comes wen he's called and gets in his kennel wen he's told and now all he does is sit and lay down I hve to physically do everthing for him pretty much. And he's also not eating! I'm taking him to the vet in the morning! But does anyone kno wht could possibly be causing this? Or tht hve similar occurancies?