actually there is a current trend to get away from recommending the car seat. while MOST children tolerate the carseat well in short spurts, some do not (they crunch down in it, especially the preemies and newborns). Imagine if you were curled into a little tight ball with a tiny airway, it could be hard for you to breathe as well. Even though he was 5 months old, this baby was very small at 5 months old (at least what I read in the thread by scanning). This right off the top of my head looks like a failure to thrive and him being in the carseat in the crib could have been / was most likely the least of his problems. It probably wasn't being in the carseat that hurt him. It was probably malnurtition or some sort of illness (that caused him to be small or more vulnerable).
But what I wanted to comment on was the carseat stuff - because of some of the comments I saw earlier in the thread. I do know that during the time I spent in the NICU as a student, we had to make sure a baby could tolerate their carseat before they left by putting them in it and checking their pulse oximetry / o2 sats for a period of time before discharge. Even so, we recommended at that NICU that parents sat in the backseat with their babies if at all possible.
Currently the places I have rotated through are recommending the use of wedges to place the baby on the side or back between the sheet and mattress along with raising the head of the crib to assist with conditions such as GER and croup during sleep. This is what we do in the hospital as well - of course, follow whatever you and your doc decide together, but that is just what I have observed in my experience.
Honestly, for most normal 5 month olds, it won't matter if they sleep in their car seat in the end because most tolerate it well. Its just those tiny stragglers who don't tolerate it well during times of sickness or because they are small that worry people.