Is the prayer of one who has a fistula from which pus is discharged valid, with the changing of the defiled dressing when necessary and performing ablution for each prayer, or by remaining in the state of ablution from the previous prayer if it has not been nullified?

If pus comes out from inside the anus, it nullifies ablution. If it comes from outside, it does not nullify it.

If you know a time slot that is sufficient for purification and prayer without the pus exiting, then wait. Otherwise, perform ablution after the time for prayer begins and pray with that ablution as much as you wish until the time for prayer ends.

Wearing a protective pad is obligatory according to the majority of scholars, but not the Malikis. It is permissible to re-apply the pad if the pus is little.

It is not permissible to abstain from ablution for the next prayer if something (pus) exits from you. Rather, you must repeat your ablution.