He remembered that he was junub and he was unable to do ghusl, so he did tayammum then he did wudoo
Question

                I left my house one morning to do some errand, and when the time for Zuhr approached, I remembered that I had had a wet dream whilst I was asleep after Fajr prayer, and I was now junub but unable to do ghusl. So I did tayammum in order to purify myself of major impurity, then I did wudoo
Praise be to Allah.If a person      has an erotic dream and emits semen (maniy), then he has to do ghusl and it      is not permissible for him to do tayammum instead, unless there is no water      available or he is afraid that he may be harmed by using water, because      Allaah, may He be exalted, says (interpretation of the meaning):