Granted, I was not there and did not see the traffic flow. Why not slow down a little and let him in? It sounds like he was going fast enough to blend into the traffic. flow. It can be a judgement call. Sometimes if I see a vehicle on the ramp, I move over to the left. Other times I speed up or slow down to allow the merge. Knowing that there are idiots like that around, I do not insist on the right of way.
With the number of idiots on the road these days, I just let them have at it. You want to drive crazy? More than happy to let you get in front of me. Too many rangers around here.