Hi tickbattler. I am sorry your little guy is going through such a hard time.
To me it sounds like he is herxing. Three to five days into treatment is when many people herx on Mepron. Increase in sweating after starting Mepron is definitely a Babesiosis symptom. He may be grumpy because he has in increase in symptoms.
It sounds like Babesiosis to me, so it is good he is being treated. I hope he feels better soon.