one more thing you mentioned about that he didn't see you.
is it a big truck becauseno idea how big US dump trucks are?
if you can't see his mirror, he can't see you.
seeing as you are the one to merge into his lane, the onus isn't on him to make sure no one is going to run into to him as you are the one making the manouever.
like i just typed on another thread, just because you got hit from the rear doesn't automatically mean it is the other person's fault.