Not much you can do really. Soak his feed and hay to keep it softer.
Mouth injuries can happen in any horse, whether in elite competition or in our own stables and pastures.
A small lip injury, split, or laceration usually doesn't require treatment because these tissues heal quickly.
Signs of a mouth infection are often subtle, such as a horse not eating. This also might be an indication that a horse has an abscessed lymph node or a draining tract under the jaw.
I hope this helps.