Discuss noel's answer to: 4 year old talks like a baby

Sounds like your son is reverting. Which is common to an extent at differents ages and emotional stages. Depending on the child's development and situations at various ages.

Is there a new baby in the family or close friend you have spending time with? 

  Have you developed a new interest or job which takes time away from your normal routine? It could be he's looking for more attention?

  What's changed recently in his little world?

Your son my feel insecure and feels he needs to prove to you he still needs you.

You could explain to him that you love to hear the things he has to say. But you can not understand him when he speaks baby talk. You need to hear his big boy voice to know what it is he is saying. Ignore the baby talk.                  Perhaps through out the day tell him how he thinks up some very wonderful and interesting ideas. How enjoyable it is to be able to communicate with him on a more grown up manner.

  Encourage a more age appropriate atmosphere. Preschool, play dates etc.

  At bed time perhaps after a bedtime story you could ask him why he feels he need to baby talk when he already speaks so well. Ask more questions without disapproving his feelings. Validate his feelings by acknowledging them.  Explain how you can see it's possible he feels the way he does. Then explain the more logical point of view. Reassure him of your love for him and the joy he brings.