Hi Jools, when we moved here the first thing we needed to do was to apply for our health cards.  In order to do this your father will need to apply for an E121 from the pensions department in Newcastle giving details of his expected date of departure from the UK.  This, when he gets here, he will then need to take to the INSS in order for them to check his eligibility with the UK.  This takes a few weeks (as is the norm) but they should then issue him with a letter to take to his doctor over here so that the health card can be issued.  At least that was the way they did it in Catalonia when we came over - whether it is the same in Aragon or not I don't know, but would imagine so.  Of course, he will still need to be able to obtain any medication he is taking whilst he is waiting for the card to be issued.  So if he can get his doctor to issue some prescriptions in order that he can obtain a supply to bring with him before he comes over it would help.  Also, if his doctor can print off the details of his prescriptions to bring with him then he will probably be able to go into a chemist and purchase over the counter if necessary.
Hope that helps.
Tony