> he has been having fits since June 2019, vets did tests to rule everything out, his almost nearly blind and as doggy dementia. My question is how much longer do we carry on
>he cries is it because he can’t see and his hearing is going his denentia or pain from the tumour how do they know,
From what you describe your dog is not happy. He has a brain tumour, is (nearly) blind, going deaf, has dementia and yes, all of those things could be reasons why he is crying. Either way, by crying he is trying to tell you that something is wrong and/or he is not happy. Added to all of that, he is having seizures.
His quality of life is the only thing that matters now and from what you say, that is compromised on a number of levels. Only you know what he is telling you (or trying to tell you). It's one of the hardest decisions you will ever have to make, but you do it with love and compassion and that is what matters.
Just keep in mind that it is better to let them go a day early, rather than a day too late.