What is incumbent upon one who had a wet dream, woke up, and a lot of semen was discharged, then had another wet dream before Fajr, so he got up, performed ghusl, changed his clothes, and then slept, and upon waking up before Fajr, he had another wet dream? And is ghusl incumbent upon him again in this situation, given the unavailability of alternative clothes?

You are required to perform ghusl if you see semen after waking up, even if you tried to prevent its emission but failed, because the obligation of ghusl is linked to the emission of semen. If you sleep after performing ghusl and have a wet dream, and semen is emitted again, then ghusl becomes obligatory again. You are permitted to pray in clothes that have been touched by semen before washing them, because it is pure according to the preponderant opinion.