I think froggy or not is more to do with how the baby is comfy rather than their size. If he will lie in the froggy position then I would put him in in that position, I think it would be better for his hips and stuff. Some babies just don't like froggy position though. If his head is wobbly, you probably need to seat him deeper into the carrier so that his head is properly supported by the headrest and tie it nice and tight. Once he's in, if you hold onto the long straps but don't tie them and then bounce up and down, he should sit down deeper into it and then tie it tight. If his head still wobbles when the headrest is properly behind him, you can always roll a muslin up behind him like a head hugger. Just keep an eye on him.