MY CAT IS DYING! What can I do? What could it be?

My cat is dying right before my eyes, I hadn't seen him properly in a few days and my dad called me downstairs and said he was dead in the garden! But it turned out he was still alive. He had loads of flys over him, and he can only walk about 1 metre or so, in a very wonky fashion before falling to the ground. It seems like he has no energy to eat or drink, and he is now just lying her beside me. He is still alive because he is breathing, but he can barely move, he won't even blink or close his eyes. he is just staring into space. What should I do to make him more comfortable in this process, and what could be be dying of? I think he is about 12 years old, that's a guess, because I don't know how old he was when we first got him, just that I've had him for about 10 years.