if you dad decides to apply for invalidity pension, he will fill out the form which is the easiest bit, he will receive a letter confirming the application within 7 days, he will then get a letter to attend medical eligibility exam in the medical referee centre nearest to him, it is only on this letter that you will be told if you are claiming any other illness ralated benefit your continued entitlement to that benefit may also be reviewed following this assessment, the medical referee will assess him for both illness benefit and invalidity pension and if he turns him down for invalidity he will also turn him down for what he is getting (illness benefit) I am going through this at the moment I am 2.5 years out of work and it seemed so easy to apply for invalidity pension get travel, medical card, etc. when you are turned down it is immediate you payments stop, then you have to appeal your case to Chief Appeals Office within 21 days furnishing documentation, letters, and history of illness you may also be asked to an oral hearing, the appeals officer can decide on your case and this appeal seems to be final, they can decide with the medical referee decision or they can offer you reduced amount depending on your disability it could be as little as 10% of the benefit you are already getting, I believe that the Ombudsman or High Court is then what is left to you should you be turned down.
My advice is in this climate is to stay with what he is getting but if he decides to go down invalidity pension route, to have a whole case supported by documentation letters from doctors, specialists, history of illness with him on the day he is being assessed by the medical referee and make sure that the he does not only offer his medical history but that he makes sure the Medical Referee takes a copy, sorry for such a long response but I am learning the hard way and would not like any sick person too have to go through the fight that I have on hand now to get my benefits and recognition of my illness.