Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
It is common for older cats to develop a hyperthyrid condition that will cause yoru old guy to lose weight even though he is eating well. The fur loss could be related to this or he has developed an allergic reaction that has caused thsi hair loss. For this, give him Benadryl at 1 mg per pound every 12 hrs. If this does not help, your vet will want to run blood work to determine the cause for his issues. This could also be caused by hormonal issues.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, Dr. Loretta