There are several possibilities. If he feels hot to you, he may have a fever, so I would recommend taking his temperature, if you can. Use a digital thermometer and do it rectally. Normal is between 100.5 and 102.5. The yellow urine could mean either that it is very concentrated, from his being dehydrated, or that he is passing excessive bilirubin into his urine. Being dehydrated is not good, but excessive bilirubin could be worse. It could mean that he has a problem with his liver or that his body is breaking down his red blood cells (the byproduct of which is bilirubin). The weakness you describe is not good--It means that he is quite ill, but is not a sign that points toward a particular illness. I worry that you said that his stomach feels tight. He could have fluid building up in his abdomen for some reason, or he might have abdominal pain, causing him to tighten up his abdomen. I would highly recommend that you take him to your vet as soon as possible. There are several possible illnesses that would benefit from early treatment: some of the infectious diseases (like Leptospirosis), a type of anemia called Immune Mediated Hemolytic Anemia, or several different liver and gall bladder problems. If he is an older dog, it is also possible that he could have a tumor of some sort. Another thing you could check, when you are looking him over, is the color of his gums. If they look pale, he might be anemic. If they look yellow-tinged or orange, he could be jaundiced (i.e., bilirubin builing up in his tissues). If they are brick-red, he could be in early shock. Please take him to your vet as soon as you can.
If you need to ask me any more questions, I'll be here. I will be happy to help.