Our dentist also has a paste that they claim has no allergens.  It is marshmallow flavor (hence the white).  Must has sugar in it.  My kids have never reacted to it.  I would just be ridiculous about reminding them of your allergens.  I know... it is scary!  Under most circumstances we would never let a stranger put something into our kids mouths.  You can never be too careful.  Maybe ask to see the ingredients list.  How bad is the rice allergy?  Legally companies don't have to label it because it isn't one of the top eight, so when they say there are no allergens they may have overlooked the rice.  I might suggest going today to get some of what they will use and first try putting it on his skin.  Corn is the second one I would be leary about.  Corn is in EVERYTHING.

Take care,
Jenny