Seth must have done this somehow! He wanted to prove how "juvenile" Jacob was so he turned him into a toddler. Jacob doesn't know how he did that, but he's certain he must be right. That means Seth didn't really leave he's just waiting somewhere to see Jacob make a fool of himself.
The question is how is he going to find Seth? Did he go back to the pool expecting Jacob to go that way? Maybe head to the lobby to watch Jacob try to escape? He could even be waiting by the daycare center for Jacob to either go looking for clothes or be dragged there after getting caught "unattended".
Speaking of clothes. Should Jacob try to find something to wear? Running around dressed like an actual toddler sounds humiliating, but so does getting spotted as a pint-sized streaker.