FireLance
Legend
Could I check how old you and your friends are? Given that you're meeting after school, it seems likely that you are in your teens, but I don't want to jump to conclusions.I'm playing 3.5 D&D. Sorry for not clarifying. It's my first group. We've had about 5-6 sessions already (We do it every Wednesday after school).
I think it's important to establish first of all why your friend wants to play such a persuasive character.
If he's new to D&D, and to RPGs in general, he may be adopting a competitive mindset with respect to the game. In other words, your friend wants to "win" D&D, and he thinks that being persuasive is the best way to do so. If so, you should emphasize that there are many challenges in the game that cannot be "won" through diplomacy, and that having too specialized a character means that he will only be useful part of the time(athough very effective when he is useful). However, if he is aware of and prepared to accept this, let him play his character!
A more worrying alternative is that he could be adopting too competitive a mindset and is using the game to "beat" the other players, including you as the DM. If so, you should emphasize that D&D is a co-operative game, and that it is also his responsibility to ensure that all the other players, including the DM, are having fun. This means, among other things, that his PC has to share the limelight with the other players' PCs, and that he will not be able to control the reactions of the other players' PCs. If he is not able to adopt this mindset, then perhaps D&D is not the game for him.
Hopefully, however, he is just keen on playing an extremely persuasive character, is willing to help ensure that the other players have fun, and will not be annoyed that persuasion doesn't always work. If so, do give him opportunities to show off his persuasion skills frequently, including the occasional spectacular success, and find some way to communicate to him early when persuasion isn't going to work or isn't going to bring any additional benefits so that he doesn't get frustrated that he has little to show after spending a lot of time and effort. One good way is to use the Insight skill - a fairly low DC Insight check (at least initially) would be a good way to communicate to him in-game how far an NPC could be persuaded.