How is he with other dogs? This can be an issue with many Terriers or Terrier mixes. It would likely be a good idea to socialize him well. Start with females.

If he socializes well, work him with other dogs. Been my experience, Dogs tend to learn quicker from another Dog than they do from humans. Just be careful which Dogs you choose, they learn the bad habits and the good habits.

All of my Terriers think they are Mastiffs and take no crap from anybody. Which can cause issues with other dog owners. One of my Terriers I have to muzzle when he is working with other Dogs. If another Dog so much as gives him the bad eye, he is on him in a flash.

He killed the neighbors Sheperd dog. The neighbors dog had bitten me before, came after me one day when I had Zach with me. Zach latched onto his throat and refused to let go. I had to pry his jaws open with a stick. Some kind of gland in the Sheperds neck got infected and killed him in about ten days. It isn't the size of the dog in the fight, it is the size of the fight in the Dog that counts.

My daughters German Shorthair sniffed my Female Plummers rear end and has a bite sized notch gone from his ear.

Terriers can be ferocious and hard to socialize well.