Speech

My son's speech is fantastic (as in, he has a huge amount of vocabulary and understands everything). However, his pronunciation isn't the best and he has some words which he says consistently but don't sound anything like what they're meant to. I can understand the majority of what he says but then I'm with him most of the time. I think others can understand some of it. It can be tricky if out of context or a longer sentence. At what point is this a concern? Not sure if there are any SLTs on here...