I assume from your message that your mum is ill from cancer and is in a hospital or maybe a hospice for treatment and care. It is not unusual to react the way your brother does by not wanting to visit a parent who is ill. This could be his way of protecting himself from the hard reality of your mum being ill. So by not visiting her, he can more easily escape from it all and pretend as if things are okay. It can also be that he is afraid of his own reactions when visiting your mum. Some people are more afraid than others about showing their emotions and he may be worried about breaking down and crying in front of her and others.
It may seem to you as if your brother is taking the easy way out at the moment. However, it is important to know that he is not acting like this because he doesn't care about your mum. It is probably because he cares so much that he can't bring himself to visit her and get so close up to what's happening to her.
He may not even know why he doesn't want to visit her. We all have mechanisms in our bodies that we call defence mechanisms which are acting at an unconscious level and are therefore out of our control. They can protect us from the truth when things get very difficult to deal with. Denial is one form of these defence mechanisms and can make us slightly num and make us think that something bad is not happening and thereby keep the reality away from us, usually just for a while. This can also be a positive thing because it can help us to deal with difficult things bit by bit so that we are not overwhelmed by the sadness of it all. Usually the reality sinks in as time passes by and we are then often able to deal with it more gradually.
Your dad is right and it is very likely that your brother will regret it later if he doesn't bring himself to visit your mum. I don't know if your mum is very ill and that may have an impact on how your brother will feel later on. In any case, it would be good if your brother could be helped so that he can manage to visit your mum. It could be a good idea to talk to someone who is involved in your mum's care - maybe a doctor or a nurse where she is being treated. They will know of people who may be able to talk to your brother to help him dealing with it all. Maybe your brother could also have a look at the riprap website and read about how others are dealing with their situation and if he feels like it he can get in touch with us to talk things through.