Little boy - Delayed behaviour...?

I have a new little boy who started with me in September. He'll be 3 in December. He is the youngest of 3 siblings and was with a nanny at home since birth. He is with me for one year until he starts school. We did no transition days with him. The first few days were hard. He would run and try to get out the front door when mom left. When she did leave he would run at me and hit me. He would sit in the corner and look at the all for most of the day. He wouldn't eat much. He's always slept well (thank goodness!)

After a week he was fine and is now a very nice kid to have around. However...

He is showing no signs of ready to toilet train at all! Doesn't know when he's pooped. Will sit in a full diaper for hours. Will sit on the toilet no problem, but has never done anything in the toilet, just sits there until i tell him he's done.

He speaks well and says some words I wouldn't expect a 3 year old to know or be able to pronounce, however, he doesn't seem to comprehend a lot. If I ask him if he had fun on the weekend he says "ya" when I ask him what he did he looks at me blankly.

When we play together we will talk about colours and shapes. He knows his stuff, but will often not respond to me. If I ask him any open ended questions i get "ya"
I could ask him if he can stand on his head and he would say 'ya'.

I will read him a story and talk about some of the different things on the page. If I ask him any open ended questions. "what is the boy doing? " etc he won't answer me at all.

Any suggestions on how to open this kid up? He's a great kid, but I would expect his behaviour from an only child, not one with 2 older siblings to learn from.

I have the same child. He will be 3 on the 31st andpotty training isn't on the horizon for his parents. I've spoken of him before as he is the size of a 5 yr old and was a biter and hitter. He is a smart boy and understands most everything but I just get the "ya" as well to everything and he otherwise doesn't answer open ended questions. I am trying hard to engage him one/one with little chats with me but I just get stared at. He is a only child. The only thing that has opened him up abit are the 2 dcg's who are 16 months old. He fetchs toys for them. So I get your frustration with the lack of communication but it just is who he is right now. :)

The only thing I can think of is that with 2 older siblings, the opposite of what you would think would happen is occurring. You would expect him to be able to converse better, but PERHAPS he isn't really given a chance. Maybe all of the talking is done for him, if you know what I mean? Mom, Dad and sibling all make the decisions for him, or don't really give him a chance to provide any input.
If he seems to be understanding everything else, I would just keep at it. Continue to ask him those open-ended questions, and try not to "give" him the answer. I would bet that one day, he is just going to start chattering away, when he realizes that he has some opinions too!