
The Boy has a doll. Didi.
At the Parent/Toddler class we took this winter, there was a tub full of baby dolls. One day Myles got into it, pulled out a likely looking one and hauled it around for an hour. I watched through the window as he carried it on his hip and patted the back of its head. He offered it some rice puffs and read to it. Then he very gently laid it down and put a paperback book "blanket" over it and patted it good night.
Okay.
So the next time I was at Baby Basics (better known as "the Used Baby Store") I found a nice blue-eyed doll ($6!). It doesn't "do" anything. It requires no batteries. Its eyes don't open and shut. It doesn't pee. That was 2-3 months ago.
This week, Didi was officially noticed and made a permanent member of the entourage. Didi eats with us and travels with us and sleeps with us.
Come on, Myles, it's time to eat.
"Didi eat, too?" in a sweet, wistful, high-pitched voice.

Sure.
Didi sits in my lap (or Grandma's) while Myles works on his spoon skills. Occasionally Didi gets a bite of something. I draw the line at blueberries. Milk is okay. Yoghurt I have to think about.
Come on, Myles, let's go get some food for dinner.
"Didi come, too?"
Sure.
The other day he was almost asleep in the car when, WHAM, he was bolt upright and patting his chest. "Didi! Didi!" So I fished around in the back seat (while driving — you learn to do this) and set Didi in his lap. The Boy snugged Didi across his legs and shoulder and immediately fell asleep.
Didi is better than Santa Claus for encouraging good behavior.
"Hmm ... Didi is too close to the stove while I am cooking. You should take Didi to the couch where it is safe."

"Look at how quietly Didi is sitting. I am happy he is letting me talk on the phone."
What really gets me misty-eyed is watching Myles be "Didi's daddy." It really is a testament to Matt's dad skills. When Myles is intentionally playing with Didi, he holds the doll so carefully. He reads to it and shows it his treasured tools.
I give him feedback, "Oh, Myles, you are so gentle. You are a good daddy." Myles gives me a winning smile and scuttles out of the room chanting "Didi's daddy! Didi's Daddy!" And thumps Didi's head against the wall on the way out.