L.C.
You should start him on cereal now that he is 6 months. I'll bet that helps.
It may also be because he is so congested and is having a hard time breathing. If you don't already have one put a humidifier in his room (I'm sure you do if he had croup). Try putting a wedge under his sheet (they sell them at baby stores) so he sleeps more upright instead of flat.
Good luck!