I share the same worry as you, my daughter wasn't speaking much at that age as well as comparing to her peers. She understands us perfectly but just couldn't speak much words.
My PD did a development assessment on her at 18 months and since she could say words like mama papa and a few others, he said it was nothing to be worried for now as each child's development is different.
My daughter is now 20 months and in a span of 2 months, she is actually speaking much more already. Give your son some time, I'm sure he'll catch up in no time.
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