He clearly has a temper problem and maybe it is just road rage or maybe it's something that will reveal itself more and more with time.
I don't know him so it's hard to say. He may just be like that when driving as there are some people who only turn into dicks on the road, however, most have that temper within them.
The whole sweet then angry thing is usually a facade that wears thin over time.

You're not overreacting. Normal people don't behave that way let alone make excuses for it. He has a temper problem. He said it won't happen again, so perhaps give him the benefit of the doubt, but if/when it does happen again, you should probably think about moving on.