Here are some ideas for you. There is not one condition that might cause all of these symptoms at the same time.
Swollen leg: Bug bite, trauma, allergic reaction
Rash: Allergic reaction
Urinary issues: UTI or other urinary disorder
Red eyes: allergies, eye infection
Weight loss: Parasite problem
I would certainly suggest a vet visit at this point. Your vet might want you to try some benadryl to fight any allergic reaction that he may be having.
Let me know if you still have questions.
