This is something I wrote to another mama who asked about dad no longer being around and I thought I'd share:
"Let your son know that you and him are a family and that you don't know why his dad isn't there but that YOU will ALWAYS love him and NEVER leave. I would always kiss my son and tell him, "Nothing in the world will ever stop me from loving you and not matter what mommy will NEVER leave you."
There is nothing you can do about the dad sadly, but just show him that he has you and that you are never going away. He needs that security. Also something I think a lot of single moms have trouble doing, which is at the detriment of many children, is let them know that just them and mom IS a family. They aren't missing out, they aren't any less loved than any other kid. I have a tattoo for my son that says, "Me and You, Just Us Two." I always let him know WE were a team, any one else in our lives was just extra but all that mattered was that we had each other."
I see ao many single moms that all they're wrapped up in is finding another man. SLOW DOWN! Your kid doesn't need another dad, or a half assed dad, what they do need though is a 100% mom. You CAN be all they need, but you have to feel that yourself. You have to have that confidence so that they can have confidence in you.