Disclaimer I am not an SLP. I think you're conflating the two terms. A developmental stutter would turn into a persistent stutter. Yes, there are cases of late onset stuttering, but that's off topic. Up to 85% of children grow out of it, so there's that. And your little boy seems to enjoy conversing, and that's a good sign. An slp, one of the stuttering "experts" told me, for the first 6 months.... don't do anything. Just interact like you would with any child. If you see it worsening after that or he is getting more frustrated or avoiding, then sure, get an evaluation if you'd like.