It's possible the dog is way oversized, and not as fat as we think. Being neutered at 6 mos certainly didn't help, I'd bet. Not only is that more likely to make him fat and lazy, but while many choose to deny it, it DOES sometimes cause aggression, specifically displaced aggression.

It's hard to say if that contributed though, b/c most dogs wouldn't be that aggressive at 6mos anyway, so you can't really do a "before and after" comparison.

Could he have a thyroid disorder? Thyroid disorders are big contributors to aggression, and at his weight, that sounds possible. How is he bred? I'm guessing he's a pet, or Am. bred dog, possibly one of the really oversized lines, unless he's just enormously fat. Could we see a picture please?

How do your wife and grandsons treat him? Give us the scenarios in which problems resulted. "Went after" is hard to pinpoint. Has he broken skin? Drawn blood? Required stitches? Did it happen once or five times? What exactly was going on when it happened? Did he bite the kids or just "yell" at them? When? What were they doing?

We need details.