Having Harley neutred is your best line of defense. Since he has been doing it for a while it has probably become a habit but hopefully you can break. He is not to old at 3, I had my boyfriends Golden Retreiver spayed at 8 years old and she did fine, and neturing is a lot easier than spaying.
Having you husband do all the care will hopefully help as well.

Good luck and keep us posted.