Normal ual curiousity in preschooler/school age boys?

Is it a normal curiosity to have one boy put another boys in their mouth? I received a call from a family member last night. She stated her 5 year old adopted son was caught doing this with another boy at daycare. When she asked her son where he learned that or who showed him that, they boy said it was my 5 year old son and that it happened at a camping trip over a year ago. I asked my son and he said his cousin is lying and that never happened. My son was with another boy about 6-8 months ago and was they had their pants down checking out eachothers . I am now wondering is it past the normal "doctor" stage or curiousity stage? Should I be more concerned? I am concerned, but I am not sure where this is coming from. This is not something my son has seen in our home. Should I have him checked out by a doctor or physcologist?