Ok so you have the mei tai. To be honest, I'd keep trying him in it, cos they are usually good from 4 months in that position. Try cupping under his bum after you put him in with your hands and rearranging his legs a bit.

The ergo is a great carrier, but if you want something for the long haul, one with a higher back is good.

Another good mei tai is a silly goose one - not sure if she still makes them, but they have an adjustable bit down the bottom that you cinch in for small babies.

Hth!!!!