Neutering will not STOP the marking, sorry.
Either belly bands or you can try to start training over. Keep him on a leash near you and limit his space, when he goes where he is supposed to reward him . You need to get a cleaner to get rid of the scent , if you don't he will just mark in the same area. It's a very very hard habit to break and neutering has nothing to do with it or stopping it.
Good Luck