Anxious parent of an 8 m old displaying symptoms

I am very anxious about my son and would like some advice from parents who have been in similar positions. 
he was a good babbler till 5 m old and I had no concerns but at 5 m he became very quiet and had reduced eye contact .

at first I hoped it was just a phase as he was working on sitting up but this has not improved and it’s been almost 3 months. He is extremely quiet ...virtually silent. He protests with a grunt if displeased. Never asks for milk though sometimes wiimpers. He smiles and laughs and his eye contact is ok now. He plays with toys , sometimes responds to his name . He doesn’t imitate and if I speak to him he looks and smiles but makes no vocalisation . He also takes ages to get to sleep ! But he does generally sleep 9 hrs at night. 
his motor skills are fine. 

I have seen a paediatrician who thinks it’s too early to diagnose anything and have tried to reassure and say watch and wait . I am really worried and can’t understand why my baby is so silent and would like to ask parents if this sounds like their baby? And what can I do?