OK, I've finally shown up. On being God's blessed instrument to tell a brother that he may not be called to vocational ministry when he thinks he is... The wider ministry of the church may help here. If this guy is in a small group, he might have a couple people who are willing to be painfully honest with him.

I'd imagine this is where a regular service review would come in especially handy. It provides a ready made forum for just this kind of conversation. Set aside some time either on Sunday evening or early in the week to constructively critique everything in the service and adult ed hour. Invite everyone who led or taught on Sunday. As our friend has public teaching opportunities, invite him to the review. Encourage where you can, but be lovingly honest with him. As the comments begin to sound graciously but clearly similar, we can pray that our brother will receive the godly criticism in the love with which it is given. This way, he doesn't feel unhelpfully singled out for critique, because everyone is giving it and getting it in the context of that informal gathering. Cultivating an atmosphere of godly criticism and encouragement makes this kind of conversation more normal, less awkward, and more likely to be received well.

If the brother is still not aware, then if I have elders in this situation, I might use them here in a collective way. This would not be to gang up on the poor guy, of course. It would simply be to help him hear the same difficult (but true) thing from multiple godly men. Invite him to ask the other elders estimation of his gifts and calling. If I don't have elders in this situation, I might grab one or two other godly men in the congregation who could serve in a similar way.