I switched Michael to a 5-40 lbs carseat at about 4 months. He was miserable in his car carrier, especially since the angle wasn’t quite right. He was much happier in the carseat that he was sitting up more. I used a noodle underneath to get him at the correct angle. I suggest going to the police station or fire department and getting some help from a certified car seat specialist. They really helped me out with Michaels carseat! Good luck and I hope your little one feels better soon!