When will my son stop screaming when I put him in the bathtub?

Soon he will enjoy. Babies react differently to the initial bathing process. For most it is a transition period before they enjoy it. My first screamed terribly for the first months of bathing then loved it. My second has loved the bath since week one. Be patient and it will get better.
Repetition. Repetition is the important part of a child's early development. They go through stages of loving something and then hating something. Try to spice it up and change the bathtub routine from the normal to something fun and rewarding. See what he likes and try to make it a more enjoyable experience for him. Maybe he is just scared of water or having his head under water.