The theory that he's Brian Slater's secret child sounds realistic enough.
I wonder if that's a copy of Stacey's key, or Jean gave handed it over to him. Maybe Jean knows about Kyle but wants to keep him a secret from Stacey.
If he is Brian's, it would be the first Slater character they've introduced in 9 years (Sean being the last one). Ernie and Bert (Kat's sons) are/were only recurring.