Some background- My dog is a 13 y/o Boston Terrier and is blind (he has cataracts in both eyes and we were advised not to remove them because of his health). For the past several years, he hasn't been able to pick up weight but after an extensive amount of tests, nothing was found to be wrong. He gets grumpy but nothing excessive. He used to eat everything (once he even stole a hot sauce packet!) and loved to cuddle. He's very thin and frail, having walking problems here and there so picking him up and carrying him was common. He also has arthritis.
The problem- For the past couple days, he's been consistently quiet as in sleeping the entire day, tucked in, only crying to go out or for a snack. Before, he would sleep a day or two but then get up and play, but he hasn't moved around much. Today, we came home and he started to whimper and cry while digging in his bed, trying to get comfortable. I switched out the beds, tucked him in, even switched him to the couch and he just couldn't get comfy without crying. Then, when I picked him up to take him to get a baby aspirin, he screamed out in pain. I took him out to potty and his back legs completely gave out, meaning I had to hold his lower body up while he tried to poop. I carried him back inside and he fell down into his bed, not moving, with his back legs basically like cooked spaghetti but his front right leg wouldn't relax. I tried to give him a snack with his aspirin and he refused, as well as any other snack or water. His breathing is slightly irregular, but since he is a Boston, I'm not sure if it's super irregular or not. However, it's about 20 breaths per minute. His pulse is also quite weak. His gums and tongue are both VERY pale pink/white and he almost refuses to open his mouth. Since he is so old and isn't healthy anyways with his normal condition, we're confused on what we should do with him. We can't afford to take him and find out that he's just tired but we also can't risk ignoring any possible problems on "maybe he's just tired". Could there be something seriously wrong?