My baby is now 15 month old " 1 y and 3 month"

We live with our family so there's my two brothers and mom and dad and also a nanny, he enjoys playing with all of us and even when we have some guests or other family members coming over , everything is fine with adults, but when he sees any baby his age or a little bit older the gets so scared, he First hide his eyes with his hand or gets behind us or crowl under the bed or furniture, if the other baby attempt to get near him he starts to cry and looks so afraid.

I really don't know why or how to fix that, I tried couple play dates and I took him to kids area but still the same problem.

Any advice or explainations please🤷🏻‍♀️