The catcher's thumb is great but more of a splint for immobilizing the joint to protect against foul tips and pitches caught on the thumb side that push the thumb back awkwardly. There are other aftermarket products that are specific to the index finger issue. Google Palmguard, Glovemate or baseball Palm & Finger pad and you will find several options. That index finger issue usually extends (or originates) down at the base of the finger and into that first joint of the palm so a pad that covers that and goes up into the index finger is best.
I have found that the Palmguard glove is the most hassle-free but he has to be willing to wear what amounts to a batting glove with a built-in pad. The other pads are effective but getting them to stay in place properly has varying degrees of difficulty. If you really want to get cheap, I have also cut neoprene pads and glued them to old batting gloves. He can cut the other fingers off if he wants more natural feel.