Even I agree with the other person who's commented. A few questions you could try to answer are, Does your son have enough time to pursue his other interests(could be anything music, dance, sports, theatres, poetry)? Is he having enough leisure time away from school, studies, and other interests? If the answer is a 'yes', then it's safe to pursue these 3 subjects. If it requires an extra half hour or one hour to better understand the concepts, that's not much of an issue. But supposing the additional subject mentally wears him and is consuming time away from his passion and leisure, it is best to withdraw from one subject.