First, I would talk to the player, not to 'correct' them, or anything, but to make sure that you understand his motivations and goals. Does he want to become the "undead smiter" or did he just kind of fall into that path? How does he envision the character developing?
Second, I would share with the player some ideas about the 'undead smiter'. Here are some of my own:
If he becomes known as a Enemy of Undeath, then you might run stories with Orcus cultists and the like, as Orcus will become angered at someone destroying his creations.
If the cleric is known for being a Terror to the Undead, then intelligent undead might try to take him out first. This is NOT to punish him. The party might enjoy the whole "Undead Fight! Protect the Cleric until he NUKES 'em!" dynamic. Just as a fire wizard become the target for Trolls, the undead smiter becomes the priority target for undead.
Consider pointing him to the feats which allow him to affect Demons and Devils as if they were Undead. This can broaden his "threat range", allowing him to be more of a striker/controller against such foes.
Consider pointing out the feats which aid with radiant damage. This will make him more effective with Radiant damage against all opponents, broadening his threat range.
I would not punish a player for picking a character which is effective and fun. If, for some reason, the build really, really causes you problems in encounter design (you wanted to run atmospheric undead scenarios and now he just nukes them), then maybe you can talk him out of the build, but I wouldn't generally work against him just because he is too effective.
Disclaimer: I have the same problem. Our cleric player picked Radiant Servant. I am a bit frustrated because I think he just chose it to be the most mechanically effective, rather than picking anything that exemplified his character, or interests. It also hurts because he was fitting very, very nicely into our party as a really dependable healer, and this pulls him away from that focus. Paragon paths are hard, because they are a one shot decision. If he later realizes this is a wrong choice, there's no way to retrain it.
On the other hand, I think he might have been frustrated because one of our strikers, the party Ranger, gets so much attention for finishing off the bad guys, even though he's the one that always needs the healing. I never tried to spotlight the ranger, but the party really seems to celebrate the person who gets the kill, not the person who ENABLES the kill. Maybe being a little short on healing will get them to appreciate him more.