Good evening - I'm sorry that Simmy is't feeling well!
Unfortunately, with the vague symptoms he is showing you, it may be very difficult to tell what exactly is happening with him over the internet. Sometimes it's even difficult to find the problem with an in-person physical examination. Cats are experts at hiding their illnesses, so what you are seeing at home is sometimes only the tip of the iceberg.
The bloating does concern me, and at his age my top concern is for cancer. If there's a mass in his abdomen, sometimes the vet can feel it when they do their physical exam. Other diseases that are very common in older cats are diabetes, kidney disease, and hyperthyroidism. All of these can cause a cat to act "off" in very vague ways. If nothing obvious is found on physical examination, blood work can sort out whether diabetes, kidneys, or hyperthyroidism is the issue.
Given his age and such vague complaints, it's very likely that you're going to need to take him to the vet to get an idea of what is happening - I'm sorry that I can't be of more help without actually getting my hands on him, but I do hope that I've given you some information to help prepare for his visit, including some ideas to discuss with your vet.
Please let me know if I can help any more :)