is that it sounds like he is doing great!! My child was by no means saying those at 15 months. Emma is my first child, but the one thing I have learned is to follow my gut. If it really is just a friend or acquaintice who is insisting, and you think he is doing fine, then I would keep it as is. Have you read Libby Kumin's work? She has some really good books on speech and Down syndrome.
Lynnelle (Mom to Emma, 2 1/2)
Emma's Homepage:
http://homepage.mac.com/lsnowbarger/peanut/Menu60.html
Family Blog
http://snowbarger.blogspot.com/