Just switch him back. It really isn't okay if he swallows some of it. Too much flouride can cause dental flourosis, in it's mild form this is light discoloration of the adult teeth, and in it's extreme form, it can causes pitting, and fractures. If you have city water, there is flouride added to that, if you also use nursery water for drinking, there is flouride in that, and it also occurs naturally in well water, you would need to get yours tested to know how much.
At this point it is probably more important that he brush and go to the dentist 2 times a year, than it is that he use flouride toothpaste.
Make sure and double check all of this advice with your dentist.