His left arm seems incredibly short!
What is his head resting on? What is his right arm holding?
Do you have a reference picture of this pose? With a complicated pose like this it might even help to take a picture from yourself!

I think some of the difficult comes from the fact is that the headstone is at an angle to the character, but you're trying to draw their arms as if the headstone is parallel to their body. Since it's at an angle, one arm would be more stretched than the other, e.g.

I think it would be easiest to redraw the headstone to be parallel to the character instead, since the character is probably more important.

If you want to redraw the headstone:
Determine the headstone's vanishing point first, based on the character. Since the character and the headstone are parallel, they would have the same vanishing points. You may want to expand your canvas so that you can fit the VP on it, as it might not be within the current canvas.
Using this new VP and your existing vertical perspective guides (which presumably fit the character), draw a rectangular plane for the front face of the headstone. Then draw the back and sides, based on the same vanishing point. At this point you'll have a rectangle bounding box for your headstone, and you'll be able to carve out the curved shape (if you need help doing that in perspective, say so and I'll try to show you how to do it).

If you'd rather keep your existing headstone and redraw the character:
Since we're seeing the headstone quite a bit from the front rather than the side, then we should either see the character's torso from the back, or, if we force the camera between them and the headstone, more from the front, and they'd probably be positioned in a different place on the canvas. The angle of the head would change accordingly, we'd be able to see more of the underside of their nose, maybe even the underside of their chin. It's a lot of redrawing.

Those lines are your vertical vanishing point, going off into the sky (zenith). The additional lines I described in my previous post are the horizontal ones, which meet on the horizon (which is probably below the canvas edge).