I echo what Richard said. It could be as simple as the doc bumping up the dose. There are other drugs and cmbinations of drugs. For example, Methotrexate can be used in conjunction with Cytoxan. Also, the doc can put your Dad on monthly infusions of Cytoxan and/or Methotrexate for example, which is much stronger. There's a world of possibilities
You Dad is in a difficult position having to run his business. Is there anyone that can take over for him for a while? He might find better results if he stopped for a time. When I work my guts out as I tend to do too often I come crashing down after a few weeks and have to have some down time. Extra stress such as that can have a negative effect.
Welcome to the group! How is your Dad doing now?