It's difficult when one person's curiosity intrudes on another person's dignity.
I remember at about 4yo disappearing off under a weeping willow with the boy next door (by mutual consent) to see if girls & boys bums were the same - we were caught with our pants down by my Mum who flipped.
After she calmed down (and my sister who is 9yrs older stopped laughing at me) we talked and established a rule that no subject was taboo to talk about in the home, but I was not to try things out with other people as I may upset someone, or be hurt myself. This allowed me to find things out in a safe way, and helped establish a strong open relationship with my mother which continued all through my teens, my first sexual relationships (I asked her for advice on contraception when I was 16) and on to the present day.
I also remember a boy at school (age about 6) who used to throw our pencils under the table, then flash at you when you went to pick them up - all the girls got fed up with this and made a plan - next time he did it we stood up & laughed at him - he stopped doing this.
I'm certainly not advocating public humiliation, but I think this boy was just after a reaction - some attention - the attention he got was not what he was after, it took away all his power and control over the situation.
I'm not sure how an adult may utilise this with a child without causing too much trauma, but there may be something - a quick mooney perhaps (your bum must have lost all interest to him now) - that can diffuse the tension around the situation, make it all look silly and, hand in hand with talking, turn the thing around?
I can only imagine how this suggestion/accusation of abuse must effect how you/his father are with your boy, (suggesting an mooney may seam totally inappropriate because of this) but you must not let unjustified guilt prevent you aiding your son to explore in an appropriate manner what he needs to find out.
Good luck with the appointment in September, we'll be thinking of you all.