capita home appointment

Hi, I wonder if anyone can help my me,I'm posting on behalf of my father. He has filled his form in for pip and has got a date for a home visit.The problem he has is he goes away once a year on a trip with some pensioners Mon to Friday and the week begins on the day they have arranged his appointment on the Monday. Now i know this is trivial to the a lot of the problems many people go though at the hands of these firms but i'm really worried about him. I've noticed that we may be able to get a change to the appointment date but can anyone please advise whether it's wise to tell them why we would like to change the date to another week. I've helped him all the way through this and i'm worried sick about this and don't know if we tell them are they likely to arrange another appointment on the same week to be awkward and deny him his chance to see the health professional on a different week.I truly dont know where to turn for help. Thank you in advance to anyone who may be able to advise me how to help my dad.By the way he is 63 next year and suffers badly with chronic Psoriatic arthritis and Sciatica and also has Diabetes. He is on the waiting to begin light treatment .Last year he had 4 months twice a week light treatment but the nurses at the hospital were very helpful and allowed him to go and continued the treatment the week after he got back. He is also on the waiting list for another root block injection in the bottom of his spine to try and help with the pain he suffers with the sciatica. 'I'm sorry if this post is long but i'm really worrying about dad because his psoriasis has flared up worse than ever and i'm sure it's because of all this stress. Thanks for any advise anyone may have..
A very worried Daughter