Hell and welcome to YT glad the internet gave you us
i would wash it. Maybe he doesnt like the smell of it. Then i would make sure that it fits him right. No pulling.I would start putting it on him for a small abouts of time and work my way up to longer. Make it fun to put it on. Sit with him and the sweater and let him sniff it. Put a treat on it. He will see that the sweater gives him treats. Put it on him and then give him a treat. Do you take home out for a walk? I would put it on when he goes out. I know it could get cold where you live
. Good luck