Why is your dog crying in pain with his back at night?

I have a 6 year old Shih Poo who to be fair, he has a lot of health problem. He has Canker which is constantly being treated with drops. He also has the common problem of dry eyes as his eyes don't produce their own tears. They're also constantly being treated with drops. For about a month now he has been randomly crying, screaming, arching his back and shaking. This can happen for ages and then he'll settle again. We didn't take him to the vet to begin with because after he'd had this 30min or so episode he'd be fine again. He can jump up onto the sofa, bed etc but can't get down - like it's some kind of fear? But he used to be able to fine. A month later and he's still doing this but we still haven't taken him to the vets because it stops after a while. He'll be like this for 3 days, then stop for 2 days. We love our dog dearly and really treat him as part of the family, but because my mum's a single mum on part benefits we can't afford to take him to the vets! I just need to know if there's any other solutions or helpful tips,websites or numbers we could call before we have to go to the vets. We're even considering getting him put down as we don't feel it's fair that he's being constantly treated with eye, ears and other problems. However, I feel he's too young. Am I wrong?

Someone please help!