Similar to Henry, Silas is giving us a run for our money in the late-speech department. I need to go back and read my posts from when Henry was 16-17 months but i think we're about at the same place-- behind. At this point he has a few signs (milk, bye bye, fan, please, sometimes light) and fewer words (hot, hat, and buhbuhbuh for his bunny). Really, that's it. Frustratingly he'll seem to get a new word (past ones included, yes, kitty and cheese) and then stop using it after a couple days. I'm not sure if i'm allowed to "count" that in the number of words he should have at this age or if i'm supposed to restrict my list to only those that make it into regular use.
I started this post this morning and while we were at the pool today he started saying truck. The neat thing about it was that he was doing his normal point and grunt and whimper and get worked up routine and i said truck. He clearly was trying to imitate me and i kept saying truck and he kept practicing and now he has a word that at least to a parent clearly sounds like truck.
So maybe we won't panic after all.