Has he climbed all the way up the tree? Some cats do that, although, thankfully, mine do not. But they're perfectly happy taking down the ornaments and delivering them all over the house.

Hopefully the fleas won't become a bigger problem. Most likely, they just traveled inside 'cause it's cold outside and they want to live. Selfish bastards.

I hope he feels better! He may just need a dose of antibiotics or something. :-(