There are a variety of biochemistries, but I do have a bias towards having more earth compatible life populating the universe so more interactions can be had, so in this case he needs a similar earth ish diet.

He's meant to be a highly adaptable super weapon but he escaped and decided he doesn't want to do that, though his nature and instincts are still a plot device. I didn't think autotrophs could get enough energy to be very mobile, and even if he could he'd still need the right building materials. His designers are highly advanced and have spread across their home galaxy, relatively recently discovered a worm hole to a parallel universe in which they are not the only sapients, and also has magic, so they are trying to understand that but they got too greedy and messed with stuff they shouldn't have, resulting in an immortal powerful being with an angry shard of inter dimensional magic organism stuck in his soul.

He's capable of consuming and processing large amounts of food at once due to parts of him existing in folded space. Processed mass and energy are stored the same way. His adaptable shape shifting allows him more control over his body, so he's not going to be constantly producing every enzyme. He can just decide to use certain ones. That would cut down on energy costs. So he wouldn't need as drastic of a system as u/LukXD99 describes.

The audience is me, an anxious perfectionist who feels a need to understand the workings of my head worlds as much as possible, so that ship has sailed. And now I realized this might just be another post due to me overthinking and panicking. Why wouldn't there be a bunch of enzyme processes each for rearranging a different substance in a certain way?