I haven't heard or had any bad reaction to Triflexis when we switched Pan over (only because they stopped making his regular meds)
We have tried Interceptor and Sentinel also.
We had him on Sentinel for years until he actually caught fleas from the neighbors dog and started having an allergic reaction to the bites. At that point the vet recommended Interceptor which we really liked (but the pharmaceutical company had taken a break in production to concentrate on human meds when we needed to stock up for the year. That's when we started on Triflexis.
For now, if he has fleas, try giving him a flea bath and vacuum vacuum vacuum. My concern is that you don't want to over medicate him by giving him another dose of meds. I know that some people use lemon water on their dogs to discourage fleas, but I've never tried it myself.
I'm sure others will be along shortly and can give more advise!