A.G.
Hi,
First off, I know your frustration. My daughter often sneaks off to do #2 and doesn't tell me that she needed to go or even did something. She's a middle child so I'm sure some of it is the need for attention, though she does get an awful lot of it.
Anyways, she goes back and forth between being fully cleaned and having frequent accidents. The only thing I've tried that seemed to have worked was some positive reinforcement through "treat coupons." If she does in it the toilet, she gets her own personilzed treat coupon, good for a lollipop, candy bar, or whatever is in the cupboard at the time. If she does it at night, she sleeps with the coupon under her pillow.
It's a slow uphill climb sometimes, but perservere and hopefully, your son will have it figured out soon. Good luck to you.