I'm glad to hear you have an answer for your boy. We battle tick-borne illnesses here too from spring till fall. But once diagnosed and on doxycyclene, they improve very quickly. Keep an eye out in the future though for flare-ups...once they have it, the illness can recur. I found whenever I see the symptoms start I get them right on the medication.
One tip: It's good to give milk thistle when your dog is on antibiotics. It helps counteract liver and kidney damage, especially in an older dog. You can buy it in the drugstore and just add it to his food.