andy said
La Folia. The outcome of this thread was that he is now more aware of his left hand dropping too low especially at sitting position.

OK, great.

A few suggestions that will help with the pain.

1.  It's not really bad if he tries to play with more of the fat part of his fingers. I'm not saying that he should never play on the tip, but there is a lot to be said about the sound you get with a little more padding and the fingers won't be so bent.
2. Secondly, and this is a tricky one, allow the instrument to fall slightly deeper into the hand. This should release tension. If he is playing many hours a day, it could be great or him to use slightly different positions with his hand. It's more relaxing, and especially with the more romantic works, works well with a deeper seated hold. See Perlman in attached photos. A lot of the older great violinists play this way but variation is key.
3. This one is very important. Keep your fingers close to the string. Don't lift them as much as you are doing. This in itself will require practice but practice lifting your fingers as little as possible. Stay relaxed in your hand.

Good luck, and keep me posted.