Less AIBU more is this normal. My 5.5 year old son still talks about poo a lot. Usually as a way of trying to impress other kids. Quite often they're not. Sometimes he'll even go into a little rhyme about poo which he's made up himself while others look on quizzically. I do correct him and ask him not to.
I suppose I'm just wondering, although undoubtedly annoying, if this still falls within whats normal for a 5 year old. I Know at this point there's a big variance between a mature child and then you get those who are immature for a little longer. However you'd think he'd get the hint that he's the only one finding the poo rhymes funny!😂 He's my only child so nothing to compare to.