Sick betta

Hey guys,
Just wanted to ask your advice. My Betts has not been looking well for the past couple of weeks. It's not swimming, disinterested in food and just sitting at the bottom. The colouring and fins seem to be healthy though. We had it for about 2.5 years so perhaps it's just old age, but I was wondering if you could give me some advice.
Thanks

You should probably quarantine him in a lower level of water. I'd imagine he's having a hard time reaching the surface for air. Adding a floating plant or two (plastic will work) and raising the temp to 80 or so would be good too.

As for his illness, can you test your water and post the results for ammonia, nitrite, and nitrate? How often are you changing the water in the tank? How large is said tank and what other fish do you have in there?

Just based on the way he's laying, it might be a swim bladder problem.

Thank you for your advice!
I tested the water yesterday and all the parameters (pH, ammonium, nitrite and nitrate) in the tank were normal. The tank is quite large, and apart from betta, has several tetras and a couple of snails.
We put our betta in an isolation tank inside the aquarium overnight, however in the morning unfortunately the fish passed away Such a shame, I think you were right about bladder problem, the fish looked bloated but it's probably was too late