I agree with Mardelin, try starting out with something more light weight. I can't help from experience since I got Alice at 5 months old and in the winter so she had a hoodie on the first day I got her and was fine with it, but the other thing I can think of is - try working with him like you would teach to "come". Put it on him in the house an hold out a treat and ask him to come to you. Idk if that will work, just a thought
