My son is 4 months and I use the ergo all the time. I started using it when he was about 2 months, without the infant insert (it seems pretty pricey for what it is, and the short amount of time you can use it, so I wanted to see if it would work without it). I sat him froggy style, with his knees up and feet flat until his legs were long enough that his knees reached the edge of the carrier. One of his daycare providers has taken trainings in babywearing and said that was a fine position -- and it's one I had used in the sling with my older son. I used the new native sling with my older son but once I found the ergo, the sling never felt as comfortable and my younger son always calmed down quicker in the ergo.
With both kids, I found that I was often putting them in the sling or ergo when they were grumpy or sleepy so sometimes they just had to get out a few last cries before they got content. Usually, a rhythmic tapping on the bottom, and/or deep knee bends seemed to do the trick - it would take 2-3 minutes, sometimes. If that doesn't work, I found that a very fast-paced walk for another few minutes would usually be effective.
also, depending on his growth, remember that in just a few short weeks, he'll be big enough to peek over the top and that might help as well.
My older son just turned 3 - he didn't like being carried in the ergo at all during the last year ... but now that I carry the baby in it, I find that he thinks it's a special treat if I carry him instead. It comes in very handy if we are out in public and he starts to get grumpy or difficult to handle. I can put the older one in the ergo on my back and carry the baby in my hands. It's counterintuitive but it is MUCH easier than having the quiet baby in the carrier and the flailing toddler in your arms!