It's interesting what happens with our parents once we're adults.
My own parents were extremely strict with us - no excuses for poor grades, no rewards for good ones. We weren't spoiled, school was our first priority over everything else. Today, they make so many excuses for my sisters and one sister's 3 teenage children - very hard to sit there and realize these are the same people.
When I was 16, I learned that my parents were married 2 years later than they'd always told us. They'd gotten married because my Mom was pregnant with my oldest sister while my Dad was finishing his PhD.
I carried that chip on my shoulder for a long time - how could they punish me for lying when they'd done it to me my whole life? They thought it was best to protect us from other people at a younger age.
I personally disagree, and we've taken an honesty first approach with our kids (who are still very young). But, I'm sure we'll re-evaluate the extent to which we share all the skeletons in our closets as they age - regardless, I want to lead by example and help prevent my kids from making some of our mistakes.
In your situation, I'd honestly suggest telling him some of the truths about how you came to be where you are. He's not going to like it, and he's going to resent you. But, if you shoulder accountability for the decisions you made in his best interest, I believe he'll come around in time.
It may even be advisable for you to have a few sessions with a therapist first - I know that sounds non-constructive, but sometimes, they're unbiased eyes help us see our own lives differently than we do. Plus, I believe he'll have a hard time accepting what you're telling him as truth as your mother has a much different perspective, and he will choose to believe who nurtures him most and gives him the most freedom right now.
Good luck. Please keep us all posted on how things go.