Please don't say that...you're not dumb at all. He wasn't honest with you, and you had no way of knowing that. You aren't psychic. That is not your fault.
As far as his past goes, as I said, I am displeased (to say the least) that he lied about it...do you know how many partners? Can you verify the number? If he's been with one other partner before...I can stand to believe his story. If it's more than one, I'm not sure I'd continue seeing the boy, tbh...I know that's not what you want to hear.
All relationships are based on trust, be it friendships, relationships, or anything else. The fact he lied, even by omission (though I trust this was brought up) violates a central tenet.

He owes it to you, if you wish to continue this relationship, to work to regain that trust. That will take time...I am sure this hurts you, and no matter what he does, he is going to have to take time to regain that trust. It will not be an instantaneous movie-esque thing for either of you.
I know you love him, and I know that makes it harder when things go wrong. Don't let your heart overrule your mind to the point of indoctrination. I did that in my last relationship...it nearly ruined me.
And please, I reemphasize, don't let this jade you. Sex can be a wonderful thing when shared between two loving,
ready adults. I do think this was soon on both your parts, but I don't think it has to end you. He
MUST, however, be there to fulfill his emotional end of the bargain, and he
MUST be honest with you.
Please know if you need me my inbox remains open, and I'll be checking back here often.