Just get him the cat. He's a 10 year old kid - he's not going to suddenly develop responsible habits within the time frame you have outlined. He'll be responsible sometimes, and sometimes he won't - it's the nature of the beast. Instead, I'd have some talks about the consequences of not following through on his chores, and I'd set up some reasonable expectations and subsequent loss of privileges - but I'd get him the cat. Little boys need something to love on just as much as little girls do.
just my opinion...
just my opinion...