In general, doing three A levels is better than doing four. Taking four A levels may impose too great a burden on a student, with the result that his or her grades are below what they could have been. Students who seek to study Maths, Physics, Engineering, or Computer Science at universities with competitive entrance standards may benefit from taking Further Maths as a fourth A level. Most other students would usually be better off sticking to three subjects.
Having said this, your son may be giving up too soon. Dropping Economics after just one lesson seems a hasty decision. Dropping Psychology after an initial bad result may also be unduly hasty. Not every educational obstacle can be overcome, but many can be overcome by careful study and persistence. At the risk of stating the obvious, life isn't a series of drop-down menus, or a video game which allows re-starts. Sometimes toughing it out and staying a course is what's needed.
Media Studies is a soft A level which won't assist your son in obtaining a place at a competitive university, if that is something which he would like to do.
A discussion with your son and one or more of his teachers might assist.