Before, he might have been frightened of loud noises or large animals. But now he has the cognitive ability to find fears in creative new places -- like the bathtub. Its porcelain walls could make him feel trapped, and emptying the tub might induce images of being sucked down the drain with the bathwater.
Wary of water or soap? Try using a washcloth or a visor to protect his eyes. He may howl anyway, but singing songs and telling stories can help distract him long enough to notice that his eyes are free of suds.
Terrified of the tub? Consider bathing him in a tiny tub or a large roasting pan. As he grows more comfortable in that space, you can try to place the smaller tub into the adult-size one.
Anxious about fixtures? If he's worried about the drain, the sound of the faucet, or the shower head, try to demonstrate them to him. When he's willing, he can sit on the bathroom floor and watch you get in and try them out. You can also practice bathing his toys together -- create a car wash -- and make the space less frightening through play.