What is the ruling on someone who experienced a wet dream but does not remember whether he performed ghusl or not, nor does he remember when this wet dream occurred, and he cannot distinguish between semen and other secretions?

If you are certain about having a wet dream and the emission of semen necessitating ghusl (ritual bath), then ghusl becomes obligatory upon you. The original principle is that your obligation remains until ghusl is performed. If you do not recall when it occurred, then act based on your best judgment and make up the prayers that you know or predominantly believe you performed without ghusl. However, if you had a wet dream but did not see any semen, then ghusl is not required. Similarly, if you doubt whether what was emitted from you is semen or something else, then choose and assign it the ruling of one of them.