love birds totally sick and partner dead

before 15 days i bought a pair of love birds
yesterday I saw my lovebirds sitting at the bottom of the cage.
not flying at all. usually when try to open cage and try to touch them they fly a lot but that time they didnt fly at all. they were just walking soo slowly especially the female one, when we called a avian vet he told us to give them tetracycline 500 medicine to both of them. when feeding them they exerted most of the liquid from their cere. we thought its normal. after a few hours we saw the female one was freezed when we took her in hand we noticed she was dead. today morning we called another vet and told him what happened. he told use dont give tetracycline instead give them Vimeral we gave it to the male one who is still alive. after giving we released him in cage. just after that he started making just a little sound continuously like he was sneezing. also from that time till now he didnt open his eyes he moves a little bit but doesnt open eyes i am really worried about him. we already lost the female one who was soo energetic and active before. now we dont want to lose this buddy. he looks really soo sick from yesterday we didnt eat his food, dirnk water, nothing
please try to help me as soon as possible