On topic: This guy obviously doesn't know what he's talking about. In general, people are insecure about what they think is correct and so listen to the advice of others to help them shore up their skills. eventually, when they see the reasoning behind it and put it into effect, they gain confidence in the abilities of the thing they just did.
But then, there's guys like these. They don't build up that confidence, they have it inborn and as soon as they think something is awesome, that confidence makes it hard to sway them from that position. These guys are 1 in 1000, just common enough to make them supremely annoying.
Your best bet is to throw some math at him, or challenge his optimizing skills. If he can beat a battle cleric of equal level with a fighter, then you shut up and stop pestering him. If he loses though, he has to start actually listening to you about the way the game works. of course, he'll still throw in some stupid comments but by showing him that he's dead wrong about something it should shake his resolve and make him more receptive. As long as you're not too harsh about it, that should make things ok