All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If someone finds a trace of sperm he is obliged to perform Ghusl, and whoever does not find a trace of sperm, he is not obliged to perform Ghusl just because he had a wet dream, or smelt a smell with no trace. To conclude, one is not obliged to perform Ghusl unless he ejaculated, as the Prophet answered the woman who asked him: "Does a woman have to perform Ghusl when she has a wet dream?, and he replied: "Yes, if she sees any water (white) discharges." [Al-Bukhari and Muslim]
Allaah Knows best.