i can relate to this on 2 levels, 1 i was abandoned by my mother has a baby and i have a 8 year old son who has asked the same questions of me when i divorced his mom.

i was lied to my entire junior life about my birth mother by my father and that made me resent the people around me to the extent that i talk to noone in the "family" and has such i trust noone now, after my father died i found letters addressed to me from her, so who do i blame now? him for lying and holding back the letters or her for not continuing to send them?

in regards to the little man, explain to him in language that he will understand that his dad left his mom and not him, so he wont see as much of him has he used to, but he his still thinking of him, even if it is a small lie, if you can, arrange for a postcard to be sent to your house from the state that his dad lives in with a short message for the little man, if anything it will make the little man feel less abandoned/rejected by his father and it keeps a window open if his father does change his mind and decide to contact his children.