Did you talk to the Vet about whether or not there will be ups and downs with this diagnosis until the antibiotics are finished? And what did they tell you when the PCV dropped slightly? Did they tell you to add any supplements or advise there was anything else you could do? Really, with this diagnosis, your Vet is your friend and you need to rely on what they tell you. Of course, you should ask any questions of them that you have, especially when things seem to be going in the wrong direction. But they should be familiar with what is happening with your little guy and be able to answer them much better than we can.
K kittenmittens84 is right though in that foaming and drooling doesn't mean there is a problem with the medication other than that the cat doesn't like the taste of it. Very common. But it's critical that he gets it. And you mentioned above that perhaps it had expired or been exposed to sunlight. Did you get a newer supply? You should discuss that with the Vet also because if you are giving him Doxy that isn't up to date, it might not be effective
K kittenmittens84 is right though in that foaming and drooling doesn't mean there is a problem with the medication other than that the cat doesn't like the taste of it. Very common. But it's critical that he gets it. And you mentioned above that perhaps it had expired or been exposed to sunlight. Did you get a newer supply? You should discuss that with the Vet also because if you are giving him Doxy that isn't up to date, it might not be effective