Cat keeps going out of litter box

Can anyone help? My cat all of a sudden is going outside of the litter box. Nothing is wrong with him as I already taken him to the vet. He poops next to his food and pisses outside of his litter box. I saw him go in his litter box this morning but his butt was poking out and he pissed all over the floor. I had to push him in so he could properly go. Today he pooped outside again, I put his feces in the litter box and put him inside. He pissed a little and when he went out he meowed at me with an attitude. He doesnt meow much. He is about to turn 3 on christmas, and ive had him since he was a baby.