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!!