I'm not sure, but I doubt he'll reach a full HM. He's flaring in the pic and he's only about 100-120 degrees in terms of fin angle. Unless he was swimming quite fast in the pic, hadn't reached full flare yet, or is not yet fully mature, I'd say thats the fullest he'll be. But I'm sure somone on here could give you a much more expert opinion.
Personally though, he's a great SD, I'd still get him if it's the color you're interested in over the finnage. But that's just me as I'm not looking to breed yet.