Johanna, so sorry to read Jonah is under the weather.

I think lots of folks have covered the allergy side of things; I tend to agree with what has been said.

I noticed you mention forced hot air for heat, and the "house is sealed up tight." You may want to check the humidity level indoors. He will feel better with a humidifier in his bedroom at night. Even if it is allergies, this will help.

I have a table top "thingamabob" which shows the temperature and humidity. Bought it years ago, at Sears. When I bought this house, it had forced hot air for heating. I had a humidity unit added to it. I can adjust the humidity level from the same thermostat that I use for heat. Good humidity levels are between 40% and 60%. May be something more you wish to check into.