Conversations with toddlers

We're working on encouraging Timothy to use "real" or "big boy" words. We're at a place right now where we're having a hard time understanding him which makes him really frustrated which turns his speech into mush. Last Thursday, I had stopped at Wal-Mart for groceries and there was something I had bought that Timothy wanted. So he starts with the verbal mush.... I stopped him and said "Timothy, do you want one?". He said "Ses" (with an "S", not the "Y" sound). Then, our conversation went like this:

Me: Timothy, say "I"

Timothy: I

Me: "Want"

Timothy: Want

Me: "One"

Timothy: Two

I chuckled and said "No, Timothy. Listen to Mommy's words. 'I want one'". Can you say "I"?

Timothy: I

Me: "Want"

Timothy: Want

Me: "One"

Timothy: Two

Me: "Not quite yet, Timothy. Let's try again. Can you say "I"?"

Timothy: I

Me: "Want"

Timothy: Want

Me: "One"

Timothy, with a very confused look on his face, looks at me and says in a questioning tone, "Three?".

At least I know he can count....