Once he's certain of his footing, he... stares at the portal.
Why is there a magic portal in the middle of a random river by a tiny town, entirely unattended. Is it indestructible? Is it secret somehow? Is there some mind magic affecting everyone in the world that makes them not want to visit, or do people just not care?
Peter walks up close to the portal, slowly, ready to react to anything that... might... need to be reacted to. But nothing at all happens. When his face is inches away from the shimmering surface of the portal he raises a hand up to it and stick his... pinky finger through. It tickles a little bit.
Also, the finger is totally not visible anymore on the other side of the magic membrane.
He pulls the finger away immediately and it's still whole. Which makes him feel kind of silly for worrying but on the other hand he would feel even sillier if he had just walked through the unknown magical portal and ended up being disintegrated by it or something, so, he stands by his paranoia. ...and he also tests a whole hand, then a whole arm. You know, just to be sure.
He's... sure.
It's literally a magic portal right there and he can step through.
Well. Worst-case scenario, he dies, and while he doesn't want to die, apparently he's only existed for less than a day, it won't be a big loss to... humanity.
(Yes it will, he's the best thing that's ever happened to this world, but he shouldn't really think these thoughts.)