Good that he's eating and pooping nicely. I don't have much knowledge, but I try to keep up with tick diseases because we live in Tick Country USA. Even with a negative snap test, if the complete blood work shows anything a little wonky, some people try an IFA (titer test for antibodies). The snap test doesn't cover all the possibilities.
I think doxy works on all tick diseases except babesiosis. What is his doxy dosage? If he is having pain and the deramaxx doesn't seem to be helping, you might try another NSAID. It's common for one to work better than others in an individual dog. But of course you have to be careful of possible negative reactions to the NSAID, too.
Hugs to Izzy.