You can get a free speech evaluation thru the school district. Call the Education Service District office for your school district. This service is mandated by Federal law. If he does need any kind of therapy for a condition that would prevent him from learning they also provide that therapy without cost to you.
My grandson was not talking very much by the time he was 2 1/2 and his pediatrician told us about this service. He is diagnosed with speech apraxia and has been getting speech therapy in a school district preschool.
It is normal for children to learn some sounds later than others. Children's speech is also affected by the teeth that they do or don't have. It is quite possible that your son's speech is OK; that he's at the appropriate developmental level. An evaluation will tell you so that you can stop being concerned.
You can read about speech difficulties on the Internet. I have some addresses but I can't find them right now.