Father-in-law and 5 year old son fighting

I agree with Secret sister. People with Alzheimer's have behavioral changes due to the nature of the disease. Have you tried talking to your Dad's doctor about Depakote? It helps calm agitation and anxiety which is probably what your Dad is experiencing. It gets worse as the disease progresses.

Any "solution" that depends on someone with dementia acting differently is not going to work. The only thing that works at all is for you to act differently, so I agree with SecretSister, remove your son and do your best to make sure he knows he hadn't done anything wrong.

You can say to your father, "I won't let (son) stay with you if you yell." and if he keeps arguing, remove him.

Remove your son from his presence. It's hard to reason with an unreasonable person. I have found I can't "make" my afflicted loved ones do anything, but can redirect, distract, or leave. If one of those we're caring behaves inappropriately, we take our son out of the situation. We explain they can't help it, and that it's not his fault. Then we shower our affection on him.

With our dads, we just forgive, and love them anyway, as if they behaved well all the time. Usually they don't. We love them regardless.