You should make an appointment with the GP to be referred to an audiologist for a hearing test. Even if you think his hearing is good, he might have a small hearing loss preventing him from hearing all the sounds properly. Ask GP for a referral to a speech therapist as soop as possible.
Speak to his teacher at school to see if he/she has noticed anything about his speech and if they are worried about it. Ask for an appointment with the SENCO to see if the school has any programmes to support him, and/or any method of monitoring his issues and progress.
It is difficult to understand what my 8 year old is saying when he talks. It was always assumed that he was shy/nervous. However, after observing him with his peers, it is difficult to understand what he is saying. I've tried to watch his mouth when he speaks and have come to realise that he doesn't open his mouth enough and he is talking through his teeth. His teacher at school hasn't mentioned anything, but I'm starting to worry as I often have to ask him to repeat himself to understand what he is saying. Can anyone give any advice on this? He is 8