You should distinguish between going outside for play time and going outside to potty. Perhaps when bringing him outside to potty, have him on a leash. That way you can also bring him to the spot you want him to go at and that kill two birds with one stone. Make sure as you are going out to say "ok lets potty". Avoid saying something like "lets go outside!" that may confuse him with thinking its play time, if that makes sense. Then later on once he's better at going in the same spot, then just open the door and say "ok potty!" and when he potties, call him back inside. Don't allow him to play.

When it is time to go outside to play, then say something like "ok lets go outside!!" If he DOES potty while playing, praise him with "good potty!" etc.