So what about his speech? Unfortunately, I do not have his detailed medical history. You do point out that his slurred speech is only "sometimes," but I do not know if this is a recent change or whether he has always had some problem with articulating words. It is not clear what percentage of his speech is intelligible and how others react when he is speaking. I do assume he has never had any problem with chewing or swallowing, does not have any physical changes in his mouth and throat areas, and does not have any known neurologic problems. Any of these could suggest and contribute to a speech problem.
I would recommend discussing your concern with his pediatrician. An evaluation by a speech and language pathologist, an expert on speech, may be appropriate.