I’m trying to teach Travis is to say please but every time I tell him to say it he just nods...He’s…

I’m trying to teach Travis is to say please but every time I tell him to say it he just nods...He’s so smart when it comes to knowing how things work and where things are suppose to go..but he’s still not talking. He knows how to say mommy and no and more. And he’ll nod for yes( even though I don’t think he really understands it). He’s 19 months this week. I’m worried