Hey Linh,
glad to see your posting updates. You've positioned the head right on the upper third which is great to see but I feel that he still needs to be nudged over just a little bit more. If this was more of a concept piece I would say that where he is would be fine but I get the feeling you're going for a more finished piece. Having him stand on a third helps create a more dynamic feeling within your painting.
I know this might seem a little bit on the tedious side but I would really recommend going back and flushing out your line work for you character. The pose you have given him has a strong action dynamic to it but it also has parts of his body turned at some extreme angles, mostly his head. Remember to try and find strong reference images and to keep in mind fore-shorting.
As for your background your horizon line is positioned to high. Having the horizon to high can make your image seem more unnatural and static. and also don't worry about having foliage on both side of the image, having to much will make you piece feel cramped.
I hope this makes sense to you. These are all problems I use to have a really hard time with and still do. so don't worry to much and just take your time and keep posting
