My son is trying to switch a course, as he is going into Grade 10. The course that he originally chose was one where the teacher practiced grade deflation. His average would drop even if he put tremendous effort in. And as he wants to pursue the path of medicine, high overall averages are important. Both him and I agree he will be much less stressed out and have a higher mark if he switched to another art credit. We both hope that this happens, but there is a possibility his guidance counselor won’t switch the course. I know that American universities look at all 4 years of high school, but what grades do medical schools look at?
Medical school’s will look at college pre-req courses and grades. What you do in HS will not matter when comes to applying to Medical school. Only caveat would be if he is targeting a BS/MD program then a more rigorous HS schedule coupled with stellar grades would be the objective.
In a word, no. It only matters what you do in college, unless you have an interesting story to tell about an illness or condition that occurred in HS that propelled you forward to wanting to become a doc.
Would BS/MD programs be looking at specific marks in high school or just an overall average throughout every year?
BS/MD programs would be looking at the same things as any college: Overall GPA and individual grades for each HS course- basically whatever is reported on your transcript.
Med school application only look at college grades.
The only way they will even see them, let alone matter is if he is doing DE courses at a college. BS/MD is different.
What art class he takes in 10th grade will not matter to colleges let alone med schools (if he does not get an A in art will matter to colleges but not to med schools).
So, feel free to take the easier art class and do well. 
Some high schools have co-op classes with outside colleges that give college credit. Those will count toward your GPA and will be looked at. Regular high school classes wont be seen anywhere or matter to regular med school applications.