Does the sin of a young man who did not marry and fell into the forbidden fall upon those who refused to marry him off, despite his efforts to do so for the sake of Allah?

Marriage is a good thing that one capable of it should hasten to. If it is not possible, then one must protect oneself from sins by fasting and other means. If a sin is committed due to not being married, its sin is upon the one who commits it. The one who refused to marry you is not held accountable for your misdeed, unless he or she prevented you from marrying without a valid reason. One must beware of the whispers of Satan, and studying in mixed universities is not permissible except out of necessity.