I'm also terrible at perspective, but here's what I can tell you from staring at it for a while:

1) The perspective of the chair is off, muddling its shape.
2) Heavy's legs are out of alignment with his body, making him look like he's sitting "side saddle".

I'll try to give you some lines, but again, please keep in mind that I suck thinking in 3D space on a 2D plane.

This is a general proof of concept sketch. I drew the chair first, using straight lines to simplify things, and then "sat" the Heavy in it starting from his pelvis, building the rest of his skeleton in as I went. I didn't make any attempt to have correct proportions, so don't use my sketch as a starting point.

Here's an attempt to bluline the picture without redrawing it from scratch.

Starting from the top:
1) I suggested some minor tweaks to Heavy's skull, ear, and jawline. These are mostly just personal opinion stuff.
2) Give Heavy's neck and shoulders better definition. His right (our left) shoulder in particular is turned too far forward, in my opinion.
3) Add Heavy's manboobs so the volume of his chest is more clear, and it doesn't look like his stomach is pushing up into his ribcage (as much).
3) The new position of the chair's arms is mostly just a suggestion on my part so Heavy looks more relaxed and slouch-y.
4) As is, the book looks kind of like a styrofoam prop, so I changed the pose of the hand to have it hold the book open by the thumb and pinkey. I added more "life" to the pages of the book itself as well--I also suggest even more detail on the topmost page(s) to imply that this book has been read many, many times.
5) Similarly, Heavy's other hand might be better served into an active petting pose. Also, unless he's wearing a very form-fitting shirt, there should also be a slight gap between the sleeves and his arms.
6) I made Heavy's legs thicker and drew them slightly further apart to reduce the "side-saddle" look. The cat has also been made plumper so that it can still occupy the same ratio of space.

Thank you very much, that's very helpful, especially on his legs and hands. Looking at it now I do see where I made him look square and unnatural. I'll try to improve the sketch using your suggestions and then see how it looks.