Whenever we saw something like this in our DD at that age, we tended to find, a couple days down the line, that she had misinterpreted something she'd seen or heard in some humorous yet horrifying way, and this new idea was very much bothering her. In this case you already think something like that may have happened related to you losing your temper, but any casual remark or observation for which your DS lacks proper context could have done it.
My advice would be to break out the coloring books/Play Doh/Legos/other activity you can do together without making eye contact, make some chit-chat as you go, and see if he'll open up to you about whatever is bothering him. Car rides work well for this, too.
