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This is Dr. David
that is good your son has a good vocabularly
him repeating words in his head is part of how he learns at this age.
he is also repeating rhyming words here
is he going to preschool or kindergarden this year? they often will be doing word repeating exercises like this in class at school, repeating rhyming words
it doesn't sound like he has anything wrong neurologically with him.
this can just be a phase he is going through with learning. and he will continue to learn more words.
you can allow him to repeat these words in his head and for him to say them out loud. that is normal for him.
let me know if you have questions.