You NEED to tell your Dad. He may want to/be able to get custody of Josh as well, which sounds like it would be better for everyone concerned. Sit Josh down and talk to him. Explain what you've told us - that you respect him, that he is intelligent, and that you love him and don't want to see him throw his future away. Even if he gets angry, don't get defensive. Be serious and straightforward. Talk to Josh, then talk to your Dad.

Jonathan is on his own path right now, but there's still a chance to keep Josh from making his brother's mistakes.