In English we pretty much always say "his studies" in plural form, probably because usually we study many things at once. For example when Mark goes to school, he has math class, chemistry class, English class etc. so he studies many things. However if you wanted to say only one thing, you would still use the plural, like some classes that are called "East-Asian studies" for example. Furthermore, a study can also mean a place where you do work (not like a job, more like studying and researching), usually a personal place inside one's home. Ex: Mark is doing homework in his study. Hope this cleared up the issue for you!