This is not an uncommon reaction, and is a coping mechanism that he will give up as he begins to feel more secure about his place in the family. Keep the areas clean, and continue to integrate him into the care of his new sis and reassure him of his role and importance to the family. Don't punish, but you may try positive reinforcement (a reward like extra story time, or a sticker) when he controls the impulse.
If it continues more than several months or the injuries worsen, call his Doctor.
Also have a dental checkup done asap in case he has issues with his molars or a type of TMJ.
Good luck to you all.
I hope my information is helpful. If you have more questions, just ask! I will gladly help, otherwise please click "accept".
Positive feedback and any bonus would be appreciated. Thank you, Christy