He is actually squatting and peeing in the plant, or he is standing up, backing up to the pot and firing?? I've never seen a 6 month old kitten spraying, but I guess anything is possible. Cats are naturally attracted to the dirt in large plant pots and enjoy peeing there.
When you take him for his neuter, ask the vet to check and make sure there is no urinary tract infection as well.
In the meantime, crumple up aluminum foil and put it all around the plant, so the soil is completely covered. Cats usually really hate this and it should deter him.
As for spraying after a neuter - usually if they are neutered before they start spraying, they will not spray. Even if neutered AFTER they start, they usually stop. As I said anything is possible, but not one of my many many neutered male cats has ever sprayed following surgery.
Some FEMALE cats will spray but that too is uncommon.