It wouldn't hurt to work with him and have goals but he is a rescue from a tough situation so I would advise you to keep your goals very flexible and your expectations low. You may not have the time to get where you would like to be but at least you will be learning, too, and maybe for the next dog you have in your life. It can only make you a better trainer.
Terriers, in my experience, are different than other breeds as far as training goes. Perhaps you should enlist some advice from a professional? You can get a one hour training session to assess him and learn some training methods that may work for him. If he loves walks, before you take him out you can get him excited by using some key words like "wanna go walkies!?!?" and then when you are training him, you can use those words, then take him on the walk when you're done. He will learn that the walk is his reward especially if you are consistent. Same goes for the teeth cleaning things. Start small -- the after a sit on command -- then build up to about ten mins of training a few times per day. Always reward on a positive note. If you are having a bad day or a bad session, keep it short, work on what he knows, and end positively.
Here us an example: Working Hemis metal article - YouTube She uses mealtime but you can use the walk or the chew thing.