My sympathies to you and your Dad. Could be several reasons why the diuretic tablets arent working, one of which might be down to what type of ascites he has eg Transudative or Exudative.
He may well be a candidate for paracentesis either at the GP surgery or as an outpatient. Definitely need to consult with your GP fairly rapidly I would think. In terms of repeated fluid drainage, you wouldnt want to do it too frequently, since it can upset all sorts of body mechanisms. 5L at one session, and no more than the equivalent of 1L per day as a maximum, I would have thought. Again, your GP would be able to better inform you.
Hope you get it sorted Juna.