Is ghusl [ritual bath] obligatory after janabah [state of ritual impurity due to sexual discharge] if a person has a wet dream and semen is discharged, whether a little or a lot? And is his prayer valid if he only performs wudu [ritual ablution] after a small amount of semen is discharged?

The emission of semen necessitates ghusl (ritual bath), whether the amount is small or large. If someone prays without ghusl, while being able to perform it and having access to water, their prayer is invalid and must be repeated. Wudu (ablution) does not remove janabah (state of ritual impurity requiring ghusl). What is obligatory upon a person in a state of janabah is to perform ghusl if they are able, or tayammum (dry ablution) if they are unable.