If Pickles is to believed, Tyler has seen his mother and has some visitation with her. I don't trust Pickles, but that's what she says.
Tyler was probably put into some long-term custody arrangement with JB and Michelle; maybe even one that would be hard to break. It might take some work and money to get him out and he may prefer to live with them. His mother lost custody many years ago, and it is definitely possible that she put her life back together in the gap years.
The world doesn't see how crazy the Duggars are. If it did, they probably wouldn't have a television show. Tyler's mother may think he is safe enough and has enough food/basic needs met. She may not have had the best education herself and may not see the disadvantages to Duggars' custody like their homeschooling, etc.
Topic (Jill/Derick): I wonder if Jill is watching her family on Counting On.