It sounds like your dog has an upper respiratory infection. They are very contagious so he could have gotten it from another dog. Since you are seeing mucus from the nose, you need to see the vet to possibly get antibiotics as soon as you can.
I would also request a fecal exam to rule out parasites as the cause for the diarrhea.
Until you can see the vet, you can use pediatric nasal saline drops to help clear the mucus from the nose.
Let me know if you still have questions.