HELP!!! Please advise...

Try to carry treats with you so you can reward him directly after. Shelties ARE smart, but with dogs there is a short window period where the treat is connected to the action they did.

I attached a pencil bag to Oberon's leash that I keep filled with treats. That way I always have treats near at hand when needed, even if I forget to put some in my pocket or if what I'm wearing doesn't have pockets.
 
As a note not all intact males who are bred mark. Mine don't. They are taught early on it is not acceptable. The problem is a lot of breeders keep their dogs as kennel dogs and it is something they never learn not to do.
Neutering does not automatically eliminate the behavior. At this point it is habit and neutering alone will not break the habit.
I think you have two options. Either put a belly band on him or treat him like a puppy. Crate at night and when you can't watch him. You can even tether him to you if needed. If you can't pay attention to him put him up. Every time he comes out of the crate take him out. If he doesn't go he goes back in the crate. Wait a little while and try again. Do not give him free access to the house. He needs to learn that the only acceptable place to potty is outside. Praise him for going outside. If he slips up in the house make sure to clean it well with an enzymatic cleaner to eliminate the smell so he doesn't get in the habit of using that spot.
If you choose to undertake retraining be prepared for it to potentially be a long process. You are trying to reshape a behavior he has allowed to get away with for years.
 
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