What is the best course of action for my dog?

I own a 9 year old medium-sized schnauzer he has always been a fun-loving, active, and playful dog but as of recently he has changed dramatically. My dog has always eaten a mixture of human and dog food.Believe me, I fought with my parents and sister about him eating human food but they wouldn't listen to me. We found out a month ago he has diabetes(I knew it but they didn't want to believe me) after he was taken to the vet because his urine was sticky in nature. The vet said he had to take insulin and that he should return for his first injection next week. A week passed and after failing to come up with the money we decided we wouldn't be able to pay for his insulin injections or his regular visits to the vet( most diabetic dogs have regular visits to the vet, the vet explained this to us). Because we couldn't afford his insulin we decided to better his eating habits we strictly fed him dog food and he was fine, he was eating, he was losing weight but not dramatically, he looked fit again and had so much energy, his health was great. He later developed this scabby looking patch on his hindside, I did some research and was scared it could be mange(it wasn't). Also it was around this time that he started to look quite emaciated and had almost lost his once voracious appetite, he was still eating but not quite as much. Anyways, we were able to take him to the vet (visit alone cost us 98 dollars but it was really bothering him) he was given an antibiotic injection and a topical spray for the patch it was not mange either just a spring allergic reaction. The patch healed nicely and we thought he was going to be okay but now his health has really taken a turn for the worse. He only wishes to drink water and does not eat, my mom has bottle fed him some Pedialyte just so he has SOME type of nutrients in his system. We have tried to feed him dog food as well as people food (things like boiled rice, grain cereals, or pieces of boiled chicken, anything at all that might add some weight to his stomach). He looks very drained and sleepy all the time and has developed a sort of black film at the bottom of his eye that does cover almost the half of his eye. His worst symptoms are that he appears incredibly lethargic, i can see it in his face that he is just enervated. He sleeps most of the day because he has no energy to go for walks (I tried walking him he has to pause a lot and I realized it was uncomfortable for him so I no longer take him for walks). He also has this limp because he sleeps all day and isnt moving or being active. This is all stemming from him not eating because he has literally NOTHING in his stomach except water which he drinks by the gallon fulls so much so he will even vomit. He was eating some smallt hings up until this week but between today and yesterday I dont feel he's eaten more than a treat, the only thing he would eat. It pains me to see him like this because he has been a part of my life for 9 years (since I was 8) and we simply can't afford to have him on insulin, which may or may not be the cause of his dwindling health. We are currently trying to save up to take him to the vet to see if perhaps he has some sort of virus or it's just a result of his diabetes we simply dont have enough money to pay for a treatment so we are considering putting him down but if it's something that can be treated we hopefully can help him. I would like to hear dogowners opinions on this. Thank you for your help