Needs motivation

There's an upside to our nearly four year old, Avery's speech delay. Like just now for instance, I'm 99.9% certain he told his older brother he was "being a butt hole" several times, and his brother never even batted an eye. He gets away with murder with that speech delay of his. Then again, we have a neighbor/friend who is convinced Avery can speak, and knows his "pretend" speech delay gets him out of a world of trouble. I would completely laugh this theory off if it weren't for the time my family was at a birthday party for a friend on their ten acre farm. 
During the party Avery went missing so I went to look for him. As I walked towards a barn on the property, a man asked me, "Are you looking for your boy?" 
"Yes," I said, thinking I'd found him. 
The man added, "Yup, he come over here a minute ago and asked me if he could get up on my tractor, and I told him sure." 
I stopped mid stride. He couldn't be talking about Avery, who didn't speak so much as a word at the time, only gibberish.
"Did he say those words?" I asked, no longer thinking this was my son we were talking about. 
"Yup," the man said, "He said, 'Can I get on yur tractor?'" 
"Oh, well that's definitely not my son," I said, and was turning to leave when out pops Avery, who nonchalantly heads my way. 
I looked at the man. "Is this the boy you were talking about?" I asked, completely dumbfounded at this point. 
"Yes, ma'am," he said, smiling. "That's him." 
I looked down at Avery as we begin walking back towards the party. 
"Did you have fun on the tractor?" I asked him, half expecting a full sentence back. 
He looked up at me, smiled and said, "Ghheejshhmeegin." 
So one of two things happened here: Either Avery is the most conniving child to ever walk the planet, or we don't need a speech therapist at all, we just need to buy a tractor and make him speak full sentences anytime he wants a ride.