Brazeku said:
If you're having problems with him being slow, you can have him make the diplomacy roll FIRST, and then use the result as guidelines for what he said (okay, you got an 18, that's good, here are some ideas for things that you can say). Then give him some choices and let him fill in the blanks with actual roleplaying.
I was thinking something along the same lines. If you've ever played Neverwinter Nights or one of the similar games, when you get to NPC interactions, you get a choice of several options and you can pick which one you want to use. (i.e. "Do you 1. Bribe the guard; 2. Try to fast talk your way past the guard; 3. Try to intimidate the guard; 4. Walk away.")
This may help Player X because it will start establishing an idea of what he CAN do when he talks to NPCs. Then over time, he would hopefully start providing more intro information, like saying, "I want to convince the guard to let us go past with a bribe," before the roll is ever made.
Thinking more on it, there is a possible problem with this solution. If the problem is not that the player doesn't have any ideas but that the player is having a hard time choosing between the options they do have, then this sort of solution won't help Player X. This could be a confidence issue (too uncertain to make a decision from the options) or an articulation problem (unable to form an option into coherent thought/speech).
I agree with the idea of talking to the player outside of the game, and perhaps to the DM as well. It is important that one player not ruin the game for the other players. Now maybe the person has a problem that needs to be given some slack, but then maybe the player needs to play a character who is sometimes socializing rather than being the group's main face man. (For instance, the rogue who goes to the back streets to look for information, which is mostly done through Gather Information, but doesn't represent the party generally.)
As a follow up: Can this Player X respond to questions like, "What are you trying to accomplish?" when Player X is roleplaying? Maybe what the player needs is some further help even before the roll. If Player X can say, "I want to get into the castle," then the DM can ask the, "Do you want to bribe/bluff/whatever?" question.