Worst-case scenario, IMO, would be if you nagged him so much about it that he went “off” baths. 
Seriously, though, I suppose there’s a–IMO–very slight risk of him picking up some Bad Bacteria and getting diarrhea. Also, depending on how soapy the water is, the soap itself could give him tummy trouble.
How old is he? He’s doing it for one of three reasons.
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He’s thirsty.
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He’s curious.
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He’s rebellious.
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Get him a special drinking cup just for the bathtub, and make a big deal out of getting him a drink of clean water.
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Tell him neutrally, “Ick, we don’t drink bath water”.
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Discourage him but without making a big deal out of it.
P.S. If you’re in the habit of letting the family iguana or turtle play/bathe/swim in the tub, the kid can pick up salmonella from it.