Up to the age of 3, speech delay is quite common. Next to some speech-delayed 3yos, your child would look pretty advanced.

Phonological delay may be an early sign of dyslexia. How is your son doing with listening to and following instructions?

Also, can you understand him? If you can, then in time he will be clear to others too. It just takes a bit of time for some children to be able to speak clearly.