Has he told you why he is afraid? Is the water too cold? Does it come down too fast? It can look scary to someone so small. Try to figure out the cause ❤️

Also, give him something he really loves, like ice cream while he is bathing and play his favorite music. Keep the bathroom warm to keep the cold out

Awww. This would be so so hard. Poor little guy. Could you try a shower instead and see how he reacts?