A second opinion is not necessary. However, it can be advisable. It depends. I haven't because I am happy (mostly) with the treatment I have been getting. But others around here have gotten second opinions and have found them very positive.
The second opinion can offer additional insight into what is happening and may present different treatment options. There may be better ways to treat what he has but he may not realize this without going for a second opinion. As for bad news, well, that would be information that the cancer is worse than initially thought. But, that should be considered "good" news because that means a more appropriate treatment. It's all a matter of perspective. You want the doctors to be treating the right things in the right way.
If he does get a second opinion, he should go to one of the top research hospitals in the country. They'll have the top experts and access to the very latest in treatment options. Here is a list of the top 100 cancer hospitals in the US. http://health.usnews.com/best-hospitals/rankings/cancer
At 23 he is an adult. He probably doesn't want "Big Sis" to protect him. This is normal. Also normal is his desire not to hear more bad news. He's had quite a lot of it. Maybe you can suggest he come to WhatNext. He can ask about second opinions and probably hear from quite a lot of people about how helpful they were.