Not possible. Drugs are not transmittable through saliva. If they are, only a minute amount could be transmitted, but it would be so minute that it wouldn't show up on a drug test...There may be another reason for this reading. Maybe they got the tests mixed up or something, as I have heard of this happening.
Not at all. You cannot transmit drugs by swapping spit (pardon the vulgarism). Sometimes the tests themselves are wrong. They have a slight percentage of coming up wrong .Try a retest. Oh and do not forget to have a good heart to heart with your son first.