I'd be concerned that the ergo wouldn't last for very long because its quite short in the body - and thats the reason i don't have one (since we both have tall kids,I know what you are going through!!)

The best ones I have found for tall kids would be something like the baby hawk, and he should be able to spread his legs around you in a few more weeks - so i would hang in there if I were you. I wear Sahti with her legs spread, and she's smaller than he is, I just make sure her knees are higher than her butt, and sort of scrunch the material around the legs a bit more so its giving more support.

Of course, I don't have a baby hawk (hehe) I have beetlebums, but they are basically the same thing. Do you have the Oh Snap or the mei tai babyhawk?