It's so frustrating. He is too heavy for our Baby Bjorn (I get terrible shoulder/back ache with it now) and he hates being harnessed into anything that isn't attached to me, so buggies are instruments of torture to him(!)
I think I'm going to have a difficult month or so until he grows into it
I didn't start using it until DS was 8mo and he has been fine, so maybe it is just a size thing. I hope that is it as the ergo is wonderful - use it all the time with my 15mo and I find it comfier than our framed carrier for long walks!
Do you carry him on your hip around the house? If he is comfy like that, it is a very similar position so you may find you don't quite have him in teh right position. Do you jiggle him down in it (literally jiggle around when you have him in it) so that his bum is lower than his knees and sat right in the carrier? That is the position he is meant to be in, but I find it does require some jiggling!
Maybe try the side carry for a while, since then his legs go around your side, which should be narrower than your back. I've had no trouble carrying either of mine at any age... you'd be amazed at how comfortable babies are with their legs completely splayed! They're bendy little critters! Did your DS seem uncomfy? Could be you're just used to the "legs together" setup of the Baby Bjorn and the Ergo position just seems strange since it's so very different.
I love my ergo. DD was about 5 months when i got it and she settled in the front really easily. Second what Loopymumsy says about get straight out walking. The motion will help your DS wriggle right down so his bum is lower than knees and help him settle. Please persevere cos i think this is one of the best carriers around.