Excessive salivation can be do to several things.
One thing that often causes this is a problem in the throat. This makes it hard to swallow which causes the saliva to drop from the mouth.
This is often due to problems such as tonsillitis
A problem with a tooth can also cause excessive salivation in cats. You can not only see a problem just by looks dental x rays may be needed.
Nausea can also cause this problem, however in this case I would not expect to see him acting normally. Liver and kidney disease can also cause hypersalivation.
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Cats who are sick will not eat so the fact that he is eating yet having some difficulty I would suspect this is due to a problem in the mouth or throat.If the nose is scabbed over it is possible he has suffered an injury to the face when outside. This could have caused a tooth injury that you can not see.
Infection in the sinus can also cause them to salivate depending on the cause of the scab on the nose there may be infection brewing under the scab.
I would try to clean the nose with antibacterial soap and apply some antibiotic ointment.
You are limited in what you can do at home without knowing what the actual cause is. The important thing is to keep him eating. Canned foods baby food(chicken or turkey)pureed boiled chicken or tuna. Offering soft foods will help keep him eating . Add water to canned food the so he doe not have to do any chewing and food can be easily swallowed.I would try to keep him close to home until you can get him in to the vet.
I hope this has helped give you some possibilities for what is going on and what you can do to help him until you can get him to the vet.