I note from your posts op that your husband has already said something to his dad to let him know what he thinks of the way he is speaking to your child. My partner had to speak to his father several years ago, warning him that if he didn't change his ways, our children would come to dislike him just as my partner had grown to hate his father's father. He remained a pompous arse but managed to curb his ridiculous victorian attitude around our kids.
Fast forward several years and I am now a few weeks into being no-contact with my"father-in-law". As I had often suspected he did indeed prove to be a selfish, manipulative, arrogant bullly, who has caused an absolute earthquake in both my and my wider family's lives. My advice to you is to insist that your husband speaks to your fil. He clearly has his own issues with him, and it is up to him to deal with his father's behaviour towards your son. As long as you and your husband both feel the same way about your fil's behaviour, and present a united front, you will find a way through this. If you try and tackle your fil (and in your position I'd want to tackle him with a crow bar!) you may find that your husband suddenly can't cope with you criticising his dad.
My partner, I am sure, knows that his dad's recent behaviour is totally unacceptable. However, in my opinion, he is so influenced and I would say damaged by growing up with such a man, he cannot see things for what they really are. Your husband has told his Dad that he was damaged by him, which is a start, but never underestimate the parental tie. My partner has not supported me in what has happened, but thinks we can just move on past it all - I can't.
Please don't find yourself in a similar situation. Your husband must speak to his dad. If the next time you see your fil he can't/won't change his ways, he doesn't deserve to have any of you in his life. You chose to be with your husband, not his father, and whilst we all have to make some allowances to oil the wheels of in-law relationships, you don't have to submit to the rantings of a childish bully. I really hope that your husband can talk some sense into your fil. However, that seems unlikely, but you hold all the cards, he's your son and you decide who he sees. Good luck.