Ok here it goes.
Keep in mind that I'm not the best at anatomy but this is a fairly simple pose so it should be alright
this is my paint over
so here is what I changed.
Your anatomy has a fan art feel to it. By that I mean it looks like you are imitating a manga/anime style.
Don't get me wrong there is nothing wrong with drawing Manga/Anime but If you talked to a good manga/anime artist they will tell you that they spent years learning real human anatomy before they could correctly stylize it.
secondly you made every thing from the waist up too big. (or everything below the waist too small)
as a result its hard to tell the age, is it a 5-7 year old? is it a 12-14 year old?
and finally his left leg looks weird it almost looks like he is an amputee, it might work in a photo but in an illustration cutting the leg off like that nearly always looks weird
In my paint over I picked an age closer to the latter
I brought the shoulders down and in, fixed the collar to be in front view, scaled down the head and moved the head and neck so it was connected to the spine I did this mostly with the warp tool and the transform tool
Drew his left leg back in with a regular high opacity brush, I also fixed the placement of his elbows
How you can avoid this in the future.
Draw from life and do figure drawing, flip your canvas while working it helps you see if you put everything in the right place. and don't be afraid to look at reference photos (from life not anime or manga)
underlying form and values:
As artists/designers its out job to take a 2d surface and make it look 3d.
perhaps the easiest way to do this is with values. But first you need to understand the underlying form. A good way to do this is to think of the body as a bunch of balls squares and imagine how they would look under a spot light from different directions
Right now he looks like he could be hanging on a coat hanger to fix this I picked colors with higher and lower values and filled in some highlights and shadows.
How you can avoid this, squint! it might sound weird but blurring your vision helps you tell if an image looks flat or not if the image looks flat when you squint you need to add a more diverse value palette
Another thing that will help is is not starting with a white canvas. instead fill it with a gray color that way you need highlights and shadows so the subject does not blend in with the background.
You can also make a new layer fill it with black and set it to the color blending mode, this lets you check values without being distracted by saturation and draw lots of still lifes
I had a few more points but they will need to wait, I have class in a few.