I'm in court a lot, and I've never seen a similar lawsuit. Companies always put unenforceable terms in their contracts. Plus. OP has an actual defense.
I strongly disagree OP has an actual defense (also a lawyer FWIW) but admittedly it's not my area of expertise. I'm unconvinced that just because you haven't seen a similar lawsuit doesn't mean companies aren't winning at the pre-trial stage. In any case, given that the OP got a sign-on bonus from the new company that covers the tuition, and is thus not out of pocket a cent (I don't think the retention bonus is something he's "out" - you get a retention bonus if you stick around, that's the point of it, to incentivize not quitting in december and waiting til June - he quit in December so didn't earn the retention bonus) I'm still really confused why so many people are advising him to fight.
FIREby35, are you in magistrate/small claims court a lot? Because that's where this dollar value case could land. Also, it would go through collections beforehand. I'd be surprised if many of these were not resolved (by payment) before they even got to court -- most people probably wise up before then that they are not going to win. In addition, as you should know, most summary judgment motions do not get oral argument, so you wouldn't even see these folks in a courtroom -- it's just decided on the papers.
OP has to weigh what this fight is worth to him personally. IF the company pursues the claim and takes a hard line approach, they will win in court. The clause is not unenforceable -- this is extremely standard stuff in corporate agreements, and there is very strong presumption in favor of enforcing an unambiguous contract. Courts routinely enforce these clauses. If OP gets an attorney who does not warn him of the likelihood that the contract will be enforced and that he will end up paying, that attorney is bordering on malpractice.
For OP, I am glad he tried to fight it, appeal to empathy, and negotiate because his former employer is being a jerk, but in the end, if the company will not budge, they are "right" in the legal sense. If OP honestly could not afford to pay back the former employer, it would create real hardship for him, or he absolutely would need to pay over an extended time period, then it may make more sense to drag it out. But under circumstances where the money is not going to break his bank, and he strongly wants to just be done with that company, I personally think it is better to pay back, cut your losses, and move on, rather than dwell within the discordant relationship. It can be very emotionally draining to put up a fight, so if the cost is not too bad, then just get out of it because he's on the losing side anyway.