Also, besides the good idea Catherine gave you, be sure to break it down into small successes. In other words, reinforce with a treat very soon after he is standing properly. Don't wait for him to give you any duration. Don't wait for him to mess up. Catch it (reinforce it) before he has a chance to move. And treat very frequently as he stands. Only ask him to do this for a few seconds, then release him with your release word, which I hope you have installed. If not, start working on that too. Let him know though, that he's finished standing and make a happy fuss over him. Let him move around, take him in a circle. And start the exercise again. "Stand" (count to yourself, 1, 2 seconds, treat.) Gradually add another second of duration with you standing right there in front of him or beside him. Then break it up with your release word. You're showing him the contrast between standing how you want him to stand and permission to move. You're reinforcing or rewarding him for success...standing for 1 second, then 2, then 3.
Next you'll break that predictability up by asking for 2 seconds, 3, 1, 5, 1. Try not to ask for more than he's ready for. If he breaks up by himself without your release word, you've asked for too much too soon. Go back to where he was holding that stand and work from there again. He'll catch on in no time. It's just a matter of giving him good, concise feed-back. When he's doing what you want, tell him, "yesssss!" (or use a clicker) in a happy voice to let him know what he's doing is going to get him a treat in a second. Good luck.