I feel bad that we haven't spoken to our son in five months.
What caused you to not speak with your son for so long.
Because I have caused him grief
If you feel you have caused your son grief and that is why he is not speaking with you then you should attempt to patch things up. It is very unlikely that your son will ignore your attempts to contact to apologize. Worst case, if you need to send a letter you can also do something like that!
But I feel like he's punishing me.
Give your son some more time, he will forgive you at some point in the future if he is a good person.  You should try to make amends by doing something nice for your son and apologizing for what you did.