In order to find him first pick up this gun(I have not tried all of them but this one works well)

Next find these barrels(they are next to the start new run portal)

Jump in a barrel and aim down then shoot once, you should fly up.
After a few attempts you should find yourself significantly above the secret island.
Once the barrel despawns, spam spacebar and face the island until you reach it.
Congrats you made it!!