My ring neck dove is dying and I don't know what is wrong with him!?

I got my ring neck dove, Teek, 4 years back with a wild dove we named Hickory. They were mates until one day my little sister accidentally let them out of the cage while my ceiling fan was on and Hickory died four days after she hit the fan. Teek was depressed but he pulled through and a year later my little sister got a white female, to be Teeks mate, for her birthday. Even though she was the one to kill MY dove. But Camille, the new dove's name, had a condition that she could only lay soft eggs. But Teek and her got along perfectly and she was his new mate. well a little over 4 months ago Camille got really sick and died. quickly. It is now 4 months later and Teek was doing great until i noticed this morning he was lying on the bottom of his cage and shaking. I brought his water to the bottom of the cage and he hastily drank it down, but then stopped suddenly and started having what i can only describe as a seizure. His head started flailing around and he did a couple rolls and his head was shaking but then it just stopped. And he seemed okay. Then the same thing happened when he tried to drink again. and then when i tried to put him on one of his poles i found out he no longer had any kind of balance. He no longer coos or really moves at all. he just sits there. I love Teek more then anything and I don't want to see him die but my father doesn't want to take him to a vet because he said it would cost too much. I don't know what to do.