No, not all high whites have blazes. But he isn't showing any signs of being a high white. He is a Veriberk, veriberks are not high white.
The only thing that looks like a Veriberk that is dangerous is a banded. He isn't a Banded. You are safe with veriberks. If he throws any high whites, I wouldn't breed them back to him or to each other (on the safe side, don't breed any two high whites together), but I wouldn't worry about him specifically carrying megacolon.
The only blazes that aren't "high whites" are recessive. And even seasoned pet rat breeders won't breed two blazes together. It's asking for trouble.