I would suggest that your second step, after first dowloading the "Waiver of Repayment" form is to have your son start to make copies of all his bills for the past two or three months: utilities, grocery, gas for the car, car payment, auto/home insurance, mortgage/rent, doctor/dentist/pharmacy charges, public transportation, everything!
He will also need copies of bank statements, paycheck stubs, things like that too. Like I said, he will have to PROVE to them it is a financial hardship. If he is like most people, when he adds up his bills verses his income, he is going to wonder how he manages to pay his bills! If this is the case, then he should consider not including certain bills... as close to $ZERO$ at the end of the month without going negative seems to be the key. If he has a stack of cash in an account he might want to consider putting it somewhere "out of sight".
Now I know this sounds like a lot, and it is. But just go slow, make the copies, fill out the form (several times if necessary before you send it in), and take it one step at a time. If there is anything good about the SSA is that they do not charge interest or penalties (at this time anyway).
By the way, if you son is approved all that means is that HE doesn't have to pay it back. But expect that this "debt" will be dumped back on your ex-wife for payment. And once SSA blows the dust of her file, they will be in contact with her to repay it then. Don't know if that concerns you or not. As for the tax refund they took, I honestly have no idea if he can get that back if he wins. I was lucky enough to start the process in time to prevent that from happening.
By the way, I'm am not trying to tell you not to talk to an attorney about this, but my experience tells me it is a waste of time. I spoke to about six or seven attorneys about my wife's case. None of them were interested in taking it because they was no money to be made if we won... only the elimination of the debt, which was over $7,000 incidentally. As a matter of fact, one of the attorneys even told me there was NOTHING we could do but pay it back! I discovered the "Waiver" request on my own in my pissed-off, crazy keyboard pounding, search of the SSA website.
Also DO NOT let the SSA talk your son into making payments while they are reviewing the Waiver, no matter how small... even $10 a month. If he does, guess what? This demonstrates that it is not a finacial hardship for him to pay it back... otherwise how could he be paying them? Nice trick, huh?
It is my understanding that while his case is being reviewed, my wife's took five or six months, all collection attempts are temporariliy suspended. Although the only collection SSA does is to "steal" your tax refund, I don't believe they use collection companys of anything like that.
As I'm sure you can tell I have nothing bu the utmost contempt for the SSA. I find it disturbing that the SSA/government could come after the children of a disability recipient to pay back benefits that they determine years/decades later should not have been paid.