Marco Needs Help to see Vet!

Been trying to get Marco to the vet now for 2 months saving up and haven't managed to get the funds. Each time had to decide whether to buy groceries/pay bills or save. It really sucks because he seems to be getting worse. I managed to find a helpful place to allow such a small amount. I got half of what's needed it took some time but im really worried now. He is my best bud means a whole lot to me and he is not like he used to be and it really hurts me seeing him this way. This would get me to the goal needed to take him sooner than later. I don't even know what's truly wrong with him this is a full checkup the works. It would cost about 500$ or so they said. I would be extremely grateful not even sure how i could explain this with words. I think he's going blind too but i'm not sure! If possible would be nice to get this by the end of the month basically few weeks. SOONER THE BETTER! Thank you for willing to help thank you very much.
(---UPDATE---) I reached goal so far so I decided to up it because this would help so much and could only imagine if they find something worse or the medicine. Just got done talking to all you great people hearing sad stories. Even how someone's vet did poor job and also the vet asked for more money. I am thankful for all the help so far! (---UPDATE---) - (---UPDATE2---) Did not know could post a update another way lol so see update for recent news and thanks again! (---UPDATE2---)