Bought an Ergo online but OMG at trying to clip the strap at the back. Finding it horribly frustrating. And I have trouble swinging Matthew behind me into the Ergo for back wearing. I've only had success by putting him in front without doing the back strap and rotating him in it to the back. I also feel the fabric doesn't provide enough support for him, it doesn't seem to go up far enough. Will keep trying but may end up selling it.
Have you tried the superman toss? Throw him (not litrally) over your shoulder while leaning forward, and put him on your back, then life the ergo up by the straps. There are MANY youtube clips to help with that I back carried Ashton from about 4 months and he loves it. Always goes to sleep. The sleep hood is for their heads not the actual fabric itself