I've heard a few people say this, and I completely disagree that it is in his best interest to stop talking. In most cases this would be true, and I generally advise everyone to clam up if they are ever arrested, but in a crime of this severity and with this much clear-cut evidence, I think his only chance at being granted some lenience (for example, a plea deal to avoid the death penalty) would be to be as open and cooperative as he possibly can. It's already a slam dunk to convict him, so by talking and spilling the beans about everything he's not going to risk making things worse for himself; they're already as bad as they can get.
I think his best strategy, in fact, would be to confess everything, tell everything, answer all questions eagerly, and make a big deal about how much he regrets that he ever got involved with such a terrible, terrible plot, and how much he wishes he could take it all back, but that he made this awful decision because he was so caught up in wanting to please his brother or whatever.
Not saying that this would personally make me want to see his punishment reduced...I personally think he's irredeemable and deserves the worst. I do, however, think behaving this way would be the path most likely to result in the best possible outcome for him.