Is my pet rat dying? Please help.?

My 2 year 2 month old rat, Jelly, weighs about 300-400 gms and since it started getting warm, has begun to grow increasingly inactive. I figured it was the heat when he started falling asleep in his food-bowl right after I would give him his vegetables. He was relatively fine till this afternoon, when he stopped scarpering around as he normally does at feeding time, and just kept sleeping. I tried getting him out a few times but he just crawled back into his cage. His breathing is very erratic, and he's secreting a clear mucous-like thing from his mouth constantly. He is also not eating. He keeps pushing his food away and upturning his food-bowl. I really do not know what the matter is and I'm scared of jostling him too much. Is he dying? If so, is he in pain? Is there anything I can do to ease his pain? I live in Delhi where there are not any vets that treat pet rats. Please help me. I know he's old and probably dying, but I hate to see him in pain like this. Please please please help me.