[cypher]

I forgot to ask you what substrate you're using & if it's different to what he was on before you got him, some hams can have an allergic reaction to wood shavings & do better with a paper based substrate so that might be something to consider.
If you do think it's a respiratory infection try to get him to the vet as soon as you can, being so tiny they can take a turn for the worst very quickly & the sooner they can get meds if they need them the better.