Yeah, in Fallout 3, Dogmeat can die. He can sure take a beating before he does, but yeah, too much damage too quickly and he's a goner.

But in Fallout 4, if Dogmeat takes a ton of damage, it just stuns him for a while and you can revive him. You can even put armor on him in Fallout 4 whereas he didn't get that in Fallout 3.