hi,

My DD is almost 18mths old and she is about the same as your son,she knows mumma and babba (dad) .I would say maybe your son knows a few more words than dd,but she understands alot of words like if i tell her to close the door she does it or if i tell her to pic something up she does it and she understands sleep time,so im not too worried about it.My dd1 was even slower than dd2 and thats probaby because dd2 is picking up so much from her sis.I think that all children develop at different times and your ds sounds up to date to me