Help! My son lost the portal in the Nether and can't get back to his world [duplicate]

Since in the comments, you've mentioned that your son was playing in Creative mode, he could go in the world options and enable cheats, enable keep inventory (it'll be a game-rule setting in the world options), then go into the in-game chat and type in the command /kill @s and your son will respawn back at where his last spawn point would be.

As for the failed Nether portal, make sure that it's the right size (the minimum size for a portal is four obsidian blocks wide and five obsidian blocks tall), like this:

O = Obsidian, X = Air

O O O O
O X X O
O X X O
O X X O
O O O O

and you light the portal with a fire source of course (flint & steel, and fire charges should do alright, though ghast and blaze fireballs can also light the portal), if the construction of the portal doesn't work even after that then it's likely a temporary glitch in the game and it may be fixed in a future update.