An orthodontist should check him, but it is probably early for treatment. The American Association of Orthodontics suggests kids should be screened by age 7. This is when the front teeth, or incisors, are growing in. At this age it is helpful to assess tooth and jaw positions.
Usually, speech patterns are learned, and tooth position doesn't necessarily cause it. Without seeing you and your bite, though, it is impossible to say for sure. If you are not unhappy with your teeth and their appearance, I would speak to a speech pathologist first.