I used the clicker again for this (see, it's a very useful training tool!). First I started out by pulling on his front legs, click and treat. Then I pulled the leg that I wanted to be "up" toward me and used the other hand to push on his neck/shoulder. I only pushed him to lean as far as I could get him before he would start moving his legs or fighting to right himself. You'll get a lot of resistance and have to push fairly hard to get them to lean just a little bit, but that's okay, it'll get easier. Click and treat each time and as he becomes more comfortable try to push him farther before you click and treat.
Even though we have carpet in the bedroom I discovered that Summit was more compliant if he was on his bed (because it's softer is my guess) so I started to have him lie down on his bed before starting this exercise. In this video I finally get him all the way over into lateral and make a big fuss and give lots of treats. If at any point the dog starts to fight going all the way over, just back up a step and click and treat for getting him as far as you can. You don't want to go too long between click and treats or it's no fun for the dog. This is demonstrated well in the second video. You will note that by the end of the video I barely have to push him and he rolls right over. We're getting close to being able to introduce a command, but he has also learned a valuable lesson about yielding to pressure. Hopefully what he takes from this exercise is that giving in to being handled gets him treats, and he will be more inclined to do so.
This exercise is much easier if they will offer a down with their legs tucked under them and not a sphinx. That was another reason for the bed. Summit always does a sphinx down, except occasionally on his bed. I've been struggling with how to teach him a different command for a down that is not a sphinx, but finally figured out a lure method to teach him to settle (see here). So now I ask for the settle before asking for the roll.
From here all you need to do is consistently use the verbal command until your dog learns to associate it with the action. Don't forget to practice rolling both sides. Dogs don't generalize.