It’s always possible that Phyllis and Lenny could have encountered a wayward Morgan after the car explosion that supposedly took his life. Perhaps Phyllis treated Morgan and brought him home much like she’s doing with Sonny now.
In fact, Lenny’s reluctance to deal with Sonny could be based in part on his experience with Morgan in the past. Stranger things have certainly happened!
Perhaps Phyllis will tell Sonny all about her experiences with Morgan in the past. Of course, that probably won’t ring a bell for Sonny, since he doesn’t remember who is is let alone his children.