Keeping up sitz baths will not hurt anything, so if they make him more comfortable, why not continue them? Drainage is a good thing, when an abscess is the diagnosis.
Clear drainage is a good sign (no ongoing infection). Perianal abscesses can take a very long time to heal, as they heal gradually from the inside out. He can wash it with water as needed. The drainage is not dangerous, but if irritating, he should use a barrier cream on the external skin.
The leaking will stop when it is healed. Sometimes they never fully heal.
Sitting on a donut cushion for a prolonged time puts too much pressure in a concentrated area. It can promote swelling, reduce blood flow, and potentially cause pressure sores. When he gets uncomfortable, it is best to shift his weight or get up and walk around.
Be sure to ask these questions of his provider. He should explain to your satisfaction. Hope this helps.
Jan 