J.M.
I hate this worse than anything. All children are different as all people are. Sounds to me like your little boy is going to be just fine and is doing things he is suppose to. I have experienced this with my ex husband and 2 older kids. When my ex had his baby 8 mnths before my youngest, I heard things like that,but then I think my baby is so smart she excels in other things and she is younger, I had to explain this to my older two so they would stop. I would let your mom know, it bothers you and you would appreciate it if she didnt compare him to others.
and Pray!! Its hard when its family to confront issues like this. Be poistive and keep encouraging him!