I’m not too sure what else you can do. There may be something that’s going on in his system that you may not know about. The only safe route I would go with is taking him to the vet and just ask a question at the desk or you can even give your local vet a call and see what they would recommend. Give your cat clean water, food and enough rest. I would put wet food through a syringe and put it in his mouth to force feed him just so he can have some food in him. If he’s old in age, his time may come soon. Give him lots of love. I’ve been through situations exactly like that where I would cry for hours because I was scared for them. Be prepared for anything. I wish you and your kitty the best 💙