Without seeing him. Sometimes they will PUFF way up like that then deflate when they are going to shed, it aids in helping to break open the old skin (like hulk)
Is your guy acting sick in anyway other than the puffing neck thing? Eating, basking, active?
When they have respiratory infections, there are a few symtoms to look for, the eyes can be stuck shut, one or both, you can sometimes hear a popping sound as they breath (it will make a noise)
And when severe they will sit hear held very high pointing up, this is when they are struggling for air.
If you fear resp infection, get him to a Vet, they need antibiotics .
let us know how he's doing.
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PHEve / Eve
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