My dad also underwent chemo and radition. For a few months he looked just like his normal self with a few minor complications from it. However, he did eventually start to face more serious complications from them. I think the worst was that he got radiation to his throat and chest area and it caused such horrible irritation that he couldn’t eat. He also had neuropathy (damage to his nerves). I think it really depends on the person and the specific treatment though. If you have concerns, maybe try asking you dad or mom about what the potential effects are of his treatment? It certainly can’t hurt to try to prepare.
No matter what though, he will still be your dad. No matter what he looks like, you will love him all the same. It can be hard when they start to appear ill but they are probably insecure too. My dad was. Just spend time with him, treat him normally and enjoy every day that you can! Keep the faith and remember that this treatment is to help him!!