First and foremost, this situation will call for you to be strong and fearless with him.
Unless he accepts you as the pack Alpha, he will not respect you, nor will he want to please you for any other reason than conniving treats from you.
You will have to start by teaching him to Stay and Heel.
Once he does these two things flawlessly, then you can train him to recognize other commands, such as OFF.
'Off' is used instead of 'Down', and means simply to place all four paws firmly on the deck. It works on dogs who jump up, or who jump on furniture you'd rather they didn't jump on.
'Down' is used solely for having him lay down, preparatory for 'Stay'.
Never use Down and Off interchangably as you will merely confuse him and make him even less likely to obey or respect you in the future.
After all, if YOU cannot remember the proper commands, what reason does your dog have to do so?
BTW, if he does not accept you as the pack leader, you could be mauled and badly injured if he contests your dominance, so be sure to use a top-quality leather muzzle on him for at least the first few months so he doesn't get any nasty ideas.
You don't know if he was previously attack trained by one of your local drug-addled retards, so assume he was and take the proper precautions.
The beauty of training him in this fashion is, not only are you far safer from attack, but most dogs LOVE having their muzzle removed; so if he is acting like a jerkoff during the training session, treat him with love, as usual, but leave his muzzle on for an hour or two after a bad session...and remove it promptly after a good session, and add to the reward with some tasty doggy treats.
Trust me, he will make the connection in no time!