I see. Well lucky you. I couldn't find ANY way to make him stand so eventually I just let him lean against the 'wall' of my shelf, pretending to be ambushing someone from around a corner, he he.
The way that I found I could have him standing made him look really constipated. And I don't think anyone wants to see Garrus in that kind of position....or...do they!? ......lol no.