I've sat mine up as they've been so keen to do it, but I've also read it's better to let them get there themselves. They will, once they start moving they figure out how to push themselves up into a sitting postiion. I think it proably is better to let them figure it out themseves. Don't worry about the 7-month thing, as with all things they do stuff at different rates - he's moving, so he's still doing more or less what he should be.