Welcome to the forums.. poor guy =(
Bettas can live anywhere between 3-5 years, sometimes longer. It just depends on each individual fish. So he is on the lower end of his life span, which means he is up there and he may not tolerate as much as easily anymore.
Laying on his side is not a good thing, nor is not eating. For now, lower his water to 3-4 inches, or put him in a small container and float/tape it to the tank to keep it warm (78-82*F).
Something has happened for him to suddenly lay on the bottom of the tank. May I ask the size of the tank, and how often and the % of water change you do per week? Given you tested the water and all was fine, it probably isn't the water chemistry itself. Did you remember to add in the water conditioner? When you last did the water change, how did you acclimate him to the new water chemistry/temp?
Are there any physical signs of him being sick? Clamped fins, loss of color, bloated belly?
I don't want to say to put in meds without knowing what is wrong.. and just laying on the bottom and not eating could be a few different things. If you feel you must do something other then the lowering of water and QTing him, you could do an Epsom Salt treatment, 1 tsp of Epsom Salt (found in any store with a pharmacy such as Walmart, in the pharmacy section) per 1 gallon, daily 100% water changes, up to 10 days. Epsom works for internal issues, such as SBD (unable to swim/stay upright), which may help him.
I wish I could offer more, but without any other symptoms, it is hard to say what is wrong...