I gave him a bath yesterday and found a flea on his head!

It was alive!
We walk twice every day and I'm in Florida, any advice for flea medication? I've been using Revolution for puppies <5lbs.
This is the first time I see a flea on him, he's not scraching though, he might have gotten it from the walk before the bath, should I keep using revolution?
He's 4 months old and 4lbs
Help!!