Having perfect pitch is a different thing from vocal production. One can replicate pitches at will and still sing out of tune when singing in a sustained way with others. At least he's not arrogant about having perfect pitch (so it seems) and having vocal production problems....

Btw during the pitch matching, ask him to cup his hand behind his ear. This reflects his voice back to him.

Place him next to someone with a strong sense of pitch, in addition to everything else, and hopefully he will catch on and maybe try to match them.

Having had Asp & Aut children and young adults in choir has been a very wonderful experience.

Agreed!!

Sorry to chime in on an old thread, but in my first working band in the early 80's, I told them at first practice that I did not sing. They responded by stating that all mouths on the bandstand sing and they put a mic in front of me.

"Aim sharp!!!", I heard that a bunch in the coming weeks at rehearsals and on breaks at the gigs. Their thinking was that if you are going to not quite hit the pitch, it is less noticeable and offensive to the listener if the singer is slightly sharp as opposed to flat.

My wife and I went and saw Black Sabbath. Through the show, I was posting to FB, "Aim sharp, Ozzie!" He was a good 1/4-1/2 step flat the whole night, and the band was already tuned down A MINOR THIRD!!! to begin with. We left early. The lead singer for Rival Sons was phenomenal. His pitch was perfect.

"Aim sharp, Ozzie!"

Poor Ozzie didn't know what world he was in half the time. LOL.

Maybe you could take some advice from someone who has some personal experience with this.

I have Aspergers syndrome and I know for a fact that perfect pitch for most autistic choristers are slightly under pitch. I had to "refine" my perfect pitch and push it higher because it was causing problems in the choir.

Maybe he is trying to push the pitch of the choir lower. Ask him his thought about the rest of the choir's pitch. Maybe he thinks that it is slightly too high. Then you can help him.

I would be wary of pushing his voice, I have been told not to push as it may cause unnecessary damage for his voice.

I want to thank you for having him in your choir. I know that he appreciates it. He is likely bullied at school and this might be his only refuge from it, I know the choir was for me as well. I think that autistic children can add so much to a choir. I have began conducting at a young age and I began as a cantor at a very young age.

Just some personal experiential advice.

Breathing is so important for maintaining a pitch, especially over long stretches. I don't know how much you've already discussed this with the choir in general, but it's good for everyone. My understanding is that the correct "support" for singing is using "...the muscles of the chest, especially those of the diaphragm...to hold back the breath..." -F. Lamperti. What this means is using the abdominal muscles to hold back the breath as one sings (scientifically, you can't consciously control only the diaphragm itself: you have to use the muscles around it to influence it). This idea of holding back the breath while singing has improved my pitch significantly in the past year. I don't know if that is worth pursuing with this particular chorister, but in my schola, I wouldn't exclude anyone from learning basic techniques. Everyone achieves the level of mastery that they are able and in their own time: you just have to expose them to the concepts (and many times revisit them!). Interestingly enough, the concept of holding back the breath while singing is very simple to explain and also simple to learn, but it is very difficult to master and do all the time.

In my experience, trying to force higher pitch on one doesn't do a thing. What can help is if you have that chorister listen to everyone for a couple of pieces before joining in to help him get a sense of the pitch. This is what I did and it really helped. Keep in mind that it will never be perfect (no pun intended) but it will help a lot.