Actually doxycycline is suppose to eliminate seizures in dogs. Was you dog having seizures prior to his being put on the meds? Maybe a coincidence?
My dog was also put on doxycycline with no previous history to seizures and seized.
I'm not a vet so I can not tell you what is going on with your dog. "see a vet! ". I did some quick reading on this and it could be a lot of things causing this.( not seizures) allergic reaction to the medicine causing other problems. Or out of date medicine. Or the dog had a pre -existing ailment.(like desert kid said). Www.1800petmeds.com/Doxycycline-prod10340.html