I would hope that 5L is going to test him. But until he is tested free he could be a carrier from his grandsire.
If you are concerned I'd contact the ranch and see what they have to say. I can't imagine a place like that would blow off the potential for introducing AM into the breed. If they do blow you off, there goes their reputation. Let us know if you contact them. They have a nice prominent website.
It's not as wide spread in Red Angus as in the black ones. There are a couple of lines that have it though. This link is to the Red Angus Association information on genetic defects: http://redangus.org/genetics/defects