Hi,
I'm sorry to hear about your father's troubles. I was diagnosed with CNS lymphoma in 2011 and successfully treated, and I've been healthy since. This illness and its treatment can cause a lot of cognitive and personality changes. High doses of steroids (dexamethasone in my case) are intensely difficult - I can't begin to explain how physically and mentally destructive they are. The good news is that they eventually, slowly, wear off if one can stop taking them.
I had issues with vision (nearly blind for a time), memory, understanding, and interpreting information. When I first entered treatment I was actively hallucinating. I was also depressed, anxious and angry. I lost almost all my muscle mass and gained 40 pounds of fat. I lost all my hair. I lost all interest in sex. I lost my balance and coordination and frequently fell down. I couldn't concentrate enough to read a book or watch a TV show sometimes. I lost my job, had to quit almost all the activities I enjoyed, and was stuck at home all day. I couldn't drive because I kept getting lost.
What I'm getting at is that this disease is so disruptive to your life that it's hard to be the same person you were before.
Things that helped me were visits from friends and family (even if I didn't seem to appreciate them at the time.) I took an antidepressant drug that helped tremendously. I was glad for any opportunity to get outside or go somewhere different. I also really liked ice cream.
Don't feel bad that your father seems different. He is different. You've got to to your best to be there anyway. Good luck.
Chris