He is looking pretty unhappy...
Tagging
@Colin_T @Essjay @GaryE @TwoTankAmin in the hope that these guys who have much more experience than me have any thoughts.
You've done well with him if he's been healthy and thriving for 1.5- 2 years!
Since you haven't introduced anything new to the tank, disease is unlikely, and I can't spot any signs of infection/fungus/external problems, except for the curve to the spine is quite pronounced. So the spine wasn't like that before? But again, while I'm studying up on fish health, I'm far from an expert, so this is my opinion as a hobbyist, not professional advice.
You do water changes, and he doesn't look like a fish stressed by poor water conditions, so I'm mentally ruling that out.
I still suspect it's old age. The change in behaviour can be sudden. Every creature starts to have things go wrong if they're lucky enough to reach old age. Sadly, things can also go wrong internally at any time. Think of elderly humans, and the health issues that are common in all animals in old age. Fish can also have any organs fail, and we can't see inside to tell what's causing the problem. If the liver, kidneys or heart are starting to fail, the fish starts to struggle. Heart failure especially leads to tiredness and listlessness. The fact he's become listless, is able to swim up to the surface, but then visibly struggles as he sinks again makes me think something internally is failing, and there won't be much you can do to change that I'm afraid except care for him while he's here.
I inherited a tank from my dad a few years ago, including some already visibly elderly tetra, the last remaining survivors of their schools. So I had three glowlights, a single cardinal, one black neon, and a few others. They don't look stunning, but they're not struggling either, and have kept going for three years longer than I thought they would! So he might live for a while longer, not as active as before since it's a lot of work for him to swim.
One thing you could do is drop the water level lower, or move him to a shallower tank so he doesn't have to swim too high to reach the surface. But if you do that, remember that less water volume means much more frequent water changes. You could also use tweezers/tongs/aquascaping tongs to target feed him his food, like some defrosted frozen bloodworms/brine shrimp/etc, so he can rest on the bottom. Maybe add some indian almond leaves for him to rest on rather than the sand.
Think palliative care to keep him as comfortable as possible.
I'm sorry that I don't have a positive answer.

I could well be wrong, and hopefully one of the lovely folks I added can also give their opinions! Please update us on his progress either way?