My Oliver is on Sentinel, and we're fighting fleas too.. I do think it has kept the infestation down though, as the fleas can't reproduce. I have only found a handful on him, I rarely see any and I'm not being bitten so I am assuming the issue isn't large. I am just praying for the first frost to kill them outside as I am SURE that's where he keeps picking them up.
There are lots of all natural things you can try, peppermint oil was one and I think tree tea oil as well. Search this site to see how they should be used. I plan on getting some this weekend and trying them, so far we've been doing Dawn baths, lots of vacuuming, and pulling off the ones we can when we can get em! It's a nasty fight that will likely last months, but with some dedication and diligence it can be won.
Next year I plan to just get Frontline or Advantix when I get his shots and heartworm done in March, so hopefully we won't have to fight this battle again!!!
Good luck!
__________________ Oliver & Mya are my  |