Congrats to you and your son on that rite of passage. (No, that's not sarcasm, in spite of how it might read.) Bummer that he didn't keep a copy. Hopefully you just want/need this for documentation purposes, and not to assist with your own tax return.
If he and you need an actual full-on copy-copy of it -- and not just a summary "transcript" of it -- have him fill out the IRS Form 4506, which is accompanied by a fee of $50. There are other (more abbreviated) copies you can get that are free. [I cannot remember if this forum permits website addresses, so I'll put it in a separate post.]
I don't know if the "minor" aspect will play much of a role, other than they might be permitted to communicate with you about his record if you provide both your SSN's, AND if his stuff was supposed to be filed as dependent with your taxes, you might be able to make a case for pity and extension (but still have to pay the $50). Good luck in your quest, and I hope you and he receive the documentation before too long.
