I suggest that he's too young to start potty training. Look up "potty training readiness" on the Internet. Only start training when he shows readiness.
As to his not wanting to be near his grandfather, then just let him be away from his grandfather. Grandfather gets the natural consequences of being a bully. lol With time, if you don't force him to interact with his grandfather he will come around again.
What do you do when he cries? I suggest that you sympathize with him, even hold him to comfort him. My 2 yo granddaughter cries often and that is what we all do. Sometimes, they cry because they are feeling insecure. Remember everything is new to them. They are developing many parts of themselves rapidly.
They cry when they're mad. Sympathize. Say, "I know you really wanted to watch that show." And then, hold him, if he wants to be held. Otherwise just stay nearby until he calms down. Let him know that you're there to help him handle his big feelings.
Also, be sure that he has a schedule, gets enough to eat and enough rest. My granddaughter cries when she's hungry and/or tired. I offer her a snack and if she continues to cry, I ask her if she wants to go night, night. She nearly always nods yes.
Sometimes a change in activity helps reduce the crying. Crying can mean she's bored and needs help finding something else to do. Put away some of his toys and bring them out at such times. Rotate the toys so that you have something "new" to give him.
Try to make things appear to go his way. Give him choices and divert his attention. When he wants to do something he can't do, move him away from where he's complaining and give him something else to do. Give him a choice between 2 things frequently instead of just saying he has to do this. For example: "Do you want me to carry you to the car or do you want to walk? It's dinner time. Do you want to eat your meat first or your potatoes? It's nap time. You can take this teddy bear to bed with you or do you want to take the truck?
As for holding him on the toilet while he screamed and cried really is a cruel thing to do. It is what bullies do to those smaller than themselves. Try to help your husband realize that he cannot make his grandson go potty.
It's good to never hold a child down unless not doing so would be dangerous to the child or others. Consider how you'd feel if someone held you down. Find ways to change the dynamics. As for potty training, if you or your husband or his father try to force him to sit on the potty, the training turns into a power struggle and the child will resist learning. No one wins in a power struggle.