I think my budgie is sick

My two-and-a-half-year-old budgie is all puffed up and is acting
lethargic. All he does is sit in his Happy Hut and pop out
occasionally to grab a bite to eat. He just got back from my
relatives' house, where we put him whil we were on vacation. There is
another bird, about 6 months older than him, who he has lived with
since we got him. The older bird is fine. I don't know what to do
about my sick bird. He eats Kaytee Fiesta seed mix and Lafeber's
Parakeet Nutri-Berries.