Sounds instead of words

Gideon is now 15 months old and still hasn't said his first word yet. However, he makes sounds for things. He has sounds for asking, giving something, and hi and bye accompanied with a wave. He walks around all day saying parts of words, including mama and dada, but it doesn't have a meaning to either Brady or I. I wonder how long he will take to speak his first word, whether in toddlerese or correctly. Both my older boys had said their first word by the time they were 15 months. Ammon's first word was thank you, but it sounded like day do. Zenock's was mama, followed closely by dada.