Sadly, I'm concerned what this means for the boy's mother. That seems most likely to me, given that it sounds like the child was not neglected.
While I understand illegal immigrants' general fear about coming forward, how many mothers really want to stay in the US at the cost of their young child?
As far as age/communication, just based on the photos, I'm not sure I'd conclude he's older than my niece who is less than 2 years old (who can't say her name yet). I think the reliability depends on who made the estimate. If it was a pediatrician at the hospital, then likely reliable. But as others have said, the stress and trauma of the situation can make it irrelevant.
I wonder what "canvassing" the neighborhood consists of. I live in a very tight neighborhood, and I've been introduced to a number of neighbors, but the only neighbors I'd recognize out of context are the ones I share a duplex with. I also never answer my door. I don't even have a working doorbell. And there is no way to access my front door to knock on it. You'd have to succeed in getting my neighbors' attention, and then ask them to knock on my front door or for permission to enter. And then I'll usually answer if I hear it.
You'd have to distribute flyers to every home with the child's picture to have any possible hope of reaching everyone, but then again, this time of year many people are away for the holidays. But hopefully they've done that.
Wonder if any neighborhood cameras show his movement. I also wonder if you take him back to the neighborhood, if he would know his route home, or to a store they frequented, etc. I'm not sure what developmental age you need to be for that, but they do recognize places at that age. I would think they've already tried that.