If my analysis is correct, he should still be in slateport where he dropped you off
Once Peeko is rescued and returned to Mr. Briney, he will offer to ferry the player in his boat among Petalburg City and Dewford Town, as well as Slateport City (but only after the Letter to Steven is delivered) until they gain the Balance Badge from their father, after which he promptly disappears from all three areas
You said that you reached route 115. You cannot jump into half the route from the south so you have to enter from the north through those waterfall things. And iirc, the only way to enter those north areas is when you do that whole cable car up the mountain, beat the fire gym, go thru the desert while exiting at its northwestern area and continuing westward towards those waterfall things I mentioned earlier. Since you said you last saw him at slateport, he should still be there. You're not stuck tho since you can go thru the tunnel east of the rock gym city which takes you to a town west of the electric gym city
The guy on this page also asks something similar to this one. But the bottomline is, the guy should still be where he last dropped you off
