The start lacks weight. As he stands up, as the hand comes off the ground only his feet are on the ground but all of his weight is behind the feet, so gravity would make him fall back on his butt.
If you want him to rise fairly straight up you need to raise the butt then bring it above the feet and rise from there. It may be closer to right than it appears but from this angle it’s weight appears too far back.
Sit on the floor and stand up yourself. Think about your body position and how you move to keep balanced.
I haven’t done much animation yet, but have read a bit lately. One big tip I have got is that the good animators use video references to get their actions right. Find or make videos of the motions you want to create and study the movements, where the body weight is.
You may have heard of animationmentor.com - sounds like a great animation school, but if you go there and look in resources there are a couple of ebooks that contain most of the posts from animationtipsandtricks.com - the second ebook also has some extra pics that aren’t on the blog. There are also some webinars that you can watch, most are chats that you may or may not get much from but tips and tricks, timing and spacing, geek out with Eric Goldberg, drawn to life, and planning and workflow have some interesting content.