Why does the dumper seem to move on so quickly?

My ex dumped a month ago and im still reeling. We were together 2 years. He dumped me over text after just hours earlier saying he loved me so much and that we had a great future ahead of us.
He, by all accounts loved me... he not only said it but showed it in all sorts of ways. But since the breakup, he won't talk to me, he's cruel the very few times weve talked (all over text mind you) he's a different person. And he's not seeing anyone and even mutual friends think he's acting very strange. But from what they say, he's also starting to be somewhat back to his happy self. How can someone that says they love me dump me so callously and now act like it never happened or that i even existed. His 8 yr old son cries for me and misses me. and he can't even tell him we split. This guy has no balls to tell me what went wrong or tell his son the truth (he told him i work a lot).
ugh... as bad as it is, i still deeply love him and its not this new version of him... its the sweet gentle guy i grew to love. But why can he move on so easily?
Why does the dumper seem to move on so quickly?
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