The retirement visa might work for him but the cost for him is extremely high. Staying on a tourist visa and renewing every few months is probably his best option. Another issue is that if your future father-in-law is in poor health, he may find it difficult to find adequate, affordable healthcare here. I assume he has health insurance of some kind in Australia that many or may not be usable here in the Philippines.
I think your fiancee is probably in the best position where he is to get any needed information.