You should be able to just grab the tails of the fabric that are hanging at your side and wiggle them down (you know, that motion like if you were drying your back/shoulders with a towel?) I don't think you really have to be able to reach back to his bum necessarily- unless you need to feel for reassurance- maybe just do it infront of a mirror so you can see even if you can't feel? To keep him stable while back there hold the top rail of the fabric tight under your armpits while you wiggle the bottom rail down over his bum.
I just started hip-sliding my 8 mo old onto my back. Up until now I'd lay the wrap down on the couch, lay him in the centre, then lower my back down to him, pull him close, then stand up and just double check that his bum was still covered, never had a problem with the bottom rail sliding up.
And I assume you know this already, but just to be sure, make sure that there's enough fabric under him (and that it's tight enough) to form a seat for his bottom...you want his knees to be as high if not higher than his bum...if his legs are left to dangle without comming up to the sitting position, the bottom rail will just work its way right up and off the bum, so you want the fabric to go down over his bum, and pretty much all the way down to his knees...much more stable....if that made any sense??
Good luck mama, I hope you figure it out! What back cary are you using?
