There are a number of things that can cause rapid breathing, being nervous from fear and excitement are two of the non medical ones. On the medical side, air sac mites, upper respiratory issues be it bacterial, fungal, viral or parasitic. Internal fluid build up or any internal swelling that would put internal pressure on the air sacs. Have you seen any open mouth breathing? You could have the vet do a crop swab to check for bacteria etc., x-rays are an option but it does not sound like that is necessary if he is eating and acting normal.