We have the same sweater for Augie, and Watson wears it very well too. I think it is hard to find sweaters to cover the rump because they are long and also keeping the front feet from slipping out, anyways, he is very handsome!
Could it possibly be a lick sore? Our lab had something on her foot that looked like that. We cleaned it twice daily with peroxide and then put desitin on it. It took about two weeks for it to heal.