Although I rarely suggest this option, I think it may be best for your stress level to file an extension form for your son this year (Form 4868)
Extensions are for tax payers who do not have all of their paperwork (such as a W2 ) or have some other problem that keeps them from filing, such as yours.
Extensions are NOT valid for payment. Any tax due must be sent with the 4868, and you stated that he owed, so you obviously know how much, and you can (and absolutely should) include the $ amount due.
I really think that taking this option, filing an extension (with his payment) will help your stress level, and allow you time to see if you can figure out his user name for 2015.
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