My son doesn't talk about his time at his dads

I have shared custody of my son with my ex, but when I ask Mr. 6 did you have fun with daddy and what did you do, he shuts down and says he doesn't like to talk about it.

I also come from separated parents and remember doing the same, and kept answers simple but thats because my mum put down my dad and thought I would have a crap time. I am actually encouraging to my son, and never speak badly of his dad.

Although I have my own reservations about his dad; he has been good by teaching Mr. 6 how to ride a bike and stuff, not fantastic on education and plays computer games too much; overall they have a good relationship.

Has anyone else experienced this? How did you overcome it and help your kids?