Hello and welcome to Just Answer. I am a licensed veterinarian and would be happy to answer your questions.
Is he breathing with his mouth open or just faster?
Can you check his gums and see if they are pink and if the color comes back to them quickly when you press on them.
Well, it is a little concerning whenever a cat breaths with their mouth open, even if intermittently. His symptoms are not specific of any one particular condition and could be due to various things. Cardiac disease can lead to fluid build up in the chest and this can cause symptoms similar to what you describe, even with a normal sounds in the chest. conditions that cause anemia (like viruses or parasites) can lead to open mouth breathing as well as the cats are not circulating enough oxygen and are anxious and distressed. Either of these conditions can lead to decreased appetite and weight loss.
Since this has already gone on for a few days, I would definitely have him examined by a veterinarian. After an exam, they may recommend an xray of his chest to better evaluate his heart and lungs. If his xrays are clear, it would be time to have some bloodwork run.
I hope this helps Please let me know if you have any further questions.