Speech unclear 7 year old

[Janeyraemer]

My son was always a late talker and I would say in general his speech is less clear and vocab less broad than his peers. We have had his hearing checked when he was 5 and this came back clear. We also had a local speech therapist come and visit at home three times and she said his speech was still in acceptable boundaries

The reason for my post is that his teacher remarked at parents evening that he can be hard to understand sometimes and we needs to work on his diction.

I understand this but the question is how? What can I do more to help him?