I think the eye is out of place.

But the abs and vains are cool.

I will not comment on anatomy, that's for you to study further, but why does the eye look like a sticker on his forehead? It looks like he has no eye, he has nothing, but someone took a felt marker and drew him eye there.

He (or it) has no gums, just teeth, glued to his face?

Why did you decide to do it that way? Still not finished?

I hadn't thought about gums to tell you the truth.

The eye is a tattoo. I'm not sure how I would make it seem more so.

The left arm, the hands, and the head is not complete.

Since you have PS6 the best way to make eye to look like a tattoo is to set the image of it on the layer above with multiply blending as an option for that layer.
To look more convincing and natural, you should add texture of a skin to head of a demon (that way it will be shown on eye tattoo), then when adding more lighting to forehead, add also a shine on the eye tattoo, then blur some edges of the eye (not all) and dissolve them a bit (maybe by few strokes with very tiny eraser tool).
I recommend adding some more of a tattoo drawing around the eye (something that looks tribal maybe?). That should work.

Tattoos are part of the skin and they are more shiny then skin itself (well, depends how old are they), but anyway, this details will make it work.

I can show you with example if you let me tamper with your original image...

well, you could put more tatoo/veins on his chest

Yeah I was thinking of something like the Illuminati symbols, something like the design and shape or something.

I don't think I'll be adding any veins on the chest, possibly some form of marking.

Likely done: