What to think of this?

My ex dumped me a month ago. He hasn't told his 8 yr old son why im not around anymore. He basically tells him im working a lot due to the holidays. A mutual friend has told me this.
Also, before we split his son and i would play a little game on an app. He still plays it with me and has recently figured out there's a messaging thing on it. I just play the game with him and dont feel its my place to say anything... because i still think my ex, and i will reconcile after time apart. so last week his son messages me that he misses me so much. it broke my heart. i love that kid like my own. i wrote back saying I missed him too. and now the message part waa closed. i feel stupid i answered him, but i feel bad. its not like i said anything about his dad's and my relationship, just reiterating his statement to me.
Why hasn't he told his son? what does that mean for our relationship?
What to think of this?
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