Excessive mounting of my son

(Trigger warning, sexual content)

We have had our 3yo berniedoodle for about 7 months. He is not neutered. He sleeps in my 10yo son’s bed. Recently he has been mounting my son incessantly, especially in the evening and at night. He often gets an erection while doing this. It is very difficult to redirect him. He also obsessively sniffs my son’s crotch during this behavior.

Twice, I am mortified to say that he has actually ejaculated near my son (once in my son’s bed, the other time on the sofa). Both times it was because my son was very focused on a book or video game and doesn’t really notice the mounting, and then before he realized it, something was wet and he said the dog “kinda peed but it isn’t pee” (because he doesn’t know any better). We have tried tiring the dog out with long walks and outdoor play, but nothing is helping.

We are at our wit’s end, please help!