Looks excellent! I love the design you've got going, and the concept is very unique. Quite nice for a rough draft. :D
A few little pointers on the anatomy, mainly just with the motion as you've got the actual muscles down pretty good:

For the feet, I think the main thing is just to bring his right foot (our left) forward a bit to make it look more like he's kinda thrusting forwards to stop momentum (think eagle landing). Then that gives you a nice, strong, C-shaped action line (Depicted in blue) that'll flow really nicely through the creature. As for the tail (depicted in green), same thing applies, just give it a little more motion. Try to avoid harsh bends and try to keep it pretty flowy. Then I'd just also add a bit more curviness and flowy-ness to the wings as well, to keep it looking elegant and natural. :3
But, all in all, it's looking quite nice. Can't wait to see it progress. :D