i did this little piece as a crit for another piece, but it might be helpful for you aswell:

the main problems in your image are:
- the deltoid muscles wrap awkwardly on the arm, this is also because of some bad shaping.
- the pectoral muscles are WAY too big and are too " solid" without him flexing.
- his lats are too thin, and seem to be almost concave.
i cant crit any of the lower body because im horrible at it myself

edit: apparently i stand corrected. i couldve sworn that there are 5! ive been living a lie!
edited my little diagram.
clr: most of what i find while image searching " abdominal muscles" or " abdominal diagrams" or " abdominal anatomy" is lots of women in tight workout clothes.