I am heartbroken as well.
It's likely a combination of things, but one or more will put a person over their tolerance level. Bullying takes many forms and something that seems as innocuous as a passing comment could be all it takes or an accumulation of little comments over a long period of time. The only one that can tell you is your son. This is where the relationship between a father and son is no better suited for such things/conversations. The trick is to keep the lines of communications open and making sure they know they can talk about anything with you. It may take a very long time, but under those conditions eventually all things will be revealed.
You should go over in your head how you'll handle bullying news (if it has happened--and I hope it hasn't). I would say getting mad isn't the right response, but you are the parent and seem to have a good head on your shoulders and will figure it out.