Is ghusl obligatory, or is tayammum sufficient for one who has had a wet dream but is unable to perform ghusl? And are his prayers with tayammum valid in the event of repeated wet dreams and difficulty performing ghusl?

Firstly: It is permissible to wipe over a splint or a medical bandage. However, if the limb is sound, it should be washed with water. If washing is difficult, then it should be wiped over. Otherwise, the remaining limbs should be washed, and then tayammum (dry ablution) should be performed for the limb that cannot be washed.

Secondly: If there is great difficulty in performing ghusl (ritual bath) due to illness, tayammum may be performed instead, based on the Almighty's saying: (And if you are ill or on a journey or one of you comes from the place of relieving himself or you have contacted women and find no water, then seek clean earth and wipe over your faces and your hands. Indeed, Allah is ever Pardoning and Forgiving.) An-Nisa/ 43. In this case, whatever parts of the body can be washed should be washed, and then tayammum should be performed.