P.L.
2 1/2 hours twice a week - - sounds like a dream for both of you. He may be a little younger - but if he's been hanging with his brother (or even followig him around) the kids a little older than him shouldn't cause any issue.
If he's already exposed to his brothers germs - I'm not sure how much worse it will be a couple of days a week.
He'll get to feel more independent - and "big boy" and you'll have a little time to run errands/work out/call a friend/take a nap or anything else you want to do.
If he loves interacting with other kids - this is great.