What I took away from this panel/my explanation:
Before I say anything, I'm going to give a quick and more in-depth explanation of what bio photons are. Like Alton already said, bio photons are these little particles that are smaller than atoms and are produced by DNA from living organisms. The purpose and function of bio photons is still being theorized on, so there's sadly no scientific explanation for it yet. For bio photons to simply just vanish from an organism, the life form would literally have to die.
For bio photons to just be gone from Henmey's body, it would mean that Alton has literally been killing Henmey ever since they made that pact. Henmey is basically dead, and the only reason he's still "alive" is because Alton is doing something to keep him around (I'm assuming some sort of Michael Afton and Ennard situation). This implies that Alton may not even be alive either, which is implied by Alton saying "the more like me you've become."
That's all from me! Let me know your ideas/theories 🙏.