Ask ur hubby to talk to ur father-in-law directly and make him understand why he should not do this as the way is incorrect and is hurting his grandson. Leave the cutting to u and hubby.
Get your husband to tell him off. He can be rude without souring relationship. There's not much you can do if you want to maintain peace with in-laws.
Ask your husband to tell your fil firmly not to do it. Sometimes old folks think they know best🤦 but I guess they have good intentions.
He should be sat down and taught to listen.