Well those water parameters seem great.
Your filter sounds like it must be quite established now, so should be cycled. How long has the tank be filtered?
However, no nitrAtes in a cycled tank either mean you've done a large water change on your 2 gal recently or the fliter is not cycled. Have you noticed any ammonia or nitrIte readings at all recently as this would indicate there is no biological filtration established, although it sounds like there should be if the filter has been going for sometime. Ph is fine for a Betta.
Keep monitoring the parameters closely, because water quality is usually the cause of illness and behavioural changes. Is the tank temperature constant, fluatuations in temperature are not well accepted by bettas.
So to recap, he's about 3 years old, he has had popeye twice in the past and you got him well again, he is much less active than before but still eating well and has no other obvious symptoms?
I'm still not too sure what is going on. It could just be a simple matter of older age and slowing down and like I said being affected by those bacterial infections in the past taking its toll or it could be the start of a new problem now, but without clearer symptons it's hard to say.
I know we keep saying it, but I've learnt this myself, water quality is key and bettas don't like flutuations in environmental conditions. So maybe just concentrate on maintaining that like you are. Keep monitoring him for now.
Maybe another member will have some more ideas