Because, like his mom and dad, he is going to earn an undergraduate and a graduate degree. If he were to get married now it would significantly decrease his chance of completing his education. And his education is more important than other considerations.
100 years from now, it's unlikely that anyone will remember my name or anything I'll ever do except for the possible exception of my descendants. To believe otherwise is the height of folly and purest of vanities. Most people can't even tell you who was President 100 year ago. Therefore, I think my marriage is far more important than my education. My great grandfather was a doctor/professor of classical Greek, Hebrew and Latin. So what? I'm positive that he and his carnal and yet faithful attraction to my great grandmother are far more consequential and better appreciated by me and my children than anything he ever learned, taught or did. Is your education more important to you than your spouse? If it is, than what does that say to your son, a young man with an eternal soul, about his significance a person?
Sorry for the lecture.