Is this behavior strange?

My son and my nephew are only a few weeks apart, and now they are 2.5 years old, they have spent a lot of time together their whole lives because me and my sister are really close. My son loves his cousin and whenever they see each other, he goes running to him all excited and says ‘hi’ and is all happy but my nephew will just stand and stare and won’t do or say anything, so my son looks around at us adults like ‘is he okay?’...sometimes we tell my nephew to say hello back but he never ever does and then my son is confused. We usually have to say something like ‘let’s go play’ in order for the boys to get past the situation. We take the boys to the local playgrounds and other kids their age are interested in playing with them and they acknowledge each other but not my nephew...he just stands there and doesn’t say or do anything except look grumpy. Does anyone else think this is strange?