I do obedience with Holly, her command for sitting on my left is "close", I think the closer they are the better (except don't let him put his paw on your foot, Holly did that for a while), and "front" for sitting in front of me, again I was told to get this as close as possible (Holly is right between my feet). When she is in "close" position I can say "front" and she gets in front, then from there I can send her back to "close" or I can send her to the right, around my back, and to my left side with the command "finish". If I want her to sit when she gets back to my left side I say "close" when she is halfway their, or I just say "heel" and start walking (their is a rally-o sign where the dog goes behind the handler but
does not sit when it reachs the left)
I taught "heel" with a combination of luring, hand targetting and a lot of bending

.
I do tend to use a hand flick when saying commands (it is really hard not to), I should really stop doing that
I get her on my left with the "close" command for stays as well, in that position I can tell her to "down" or "stand" for down and stand stays.
The way I was shown to teach "front" was by sitting in a chair and luring the dog between your legs so they have to be straight (also, some dogs arn't confortable getting that close to a standing person because it is hard to not lean over them a bit). From there you stand leaning against a wall so your leaning back, then you start to stand straight. This way the dog learns (hopefully) from the start to be straight because your legs and feet guide them and they should be comfortable getting really close to the front of you.
Although the problem with this method is it didn't work so well with Holly because she is too short

, she couldn't see my face when I was sitting so she would put her paws up, but it was alright because she is comfortable having people lean over her and her butt doesn't stick out past the end of my feet that much so it is very easy for her to sit straight.
He tends to sit on his side with his feet out to the side, its adorable, but it looks bad when we are trying to work on this cue. I will try luring him right into it, and keeping the treats as high value as possible and see if that works. When you say step back do you mean stepping back to get him to follow and straighten out?
I don't know if this is true, but I'm sure I've read somewhere that some dogs sit like this because that's what they were taught when puppies. Puppies tend to sit like that and when you taught Remi to sit you probably did think it was cute (or just didn't notice because that's just how he sat) and so he learn't that.
Maybe you could try luring him into a sit (now that he's older) without asking him to sit and see how he sits, it might be straight then, I'm not sure as I've never had a large dog or a dog that sits on its side, but it might be worth a try.
With him sitting on his side, I'd use high value treats, and lure him as if you were just training him to sit, but maybe bring the treats a little higher. This seems to be common in Aussies. It is great for your stays though! Other breeds like Boxers and Whippets refuse to sit or down in a relaxed position.
I'm not sure about how good it is for stays. My obedience trainer always gets people to straighten up their dogs during stay exercises if they sit on their side. I think she said they will be more likely to break the sit-stay if they arn't straight because it will put pressure on one hip after a while and get uncomfortable (especially if sitting on concrete) so the dog will move. But I've never had a problem with this with Holly so I could be wrong.
I have seen a few dogs do down-stays lying on their side, so not lying straight probably isn't a problem.
Disclaimer: Holly is the first dog that I have trained and we haven't started to compete yet so I could be completely wrong about everything

, but I hope this post has something useful to you in it
