I wouldn't blame your company for the problem, because there is nothing illegal in keeping your son on the policy until he turns 26. Rather, I would wonder why it didn't occur to your son's HR or benefits person to ask whether or not he, as a brand new college graduate, might be claimed as a dependent in the same year.
Your son can do his tax return with TurboTax, but he will have to make a special series of entries.
Because he was able to be claimed as a dependent (even if he wasn't actually claimed), he was not eligible at all to contribute to his HSA in 2018. This is different from making an excess contribution (although TurboTax treats it in a similar way).
So he will tell TurboTax in the HSA interview that he did not have HDHP coverage at any point in 2018, and TurboTax will declare all the contributions in 2018 as excess. He will need to withdraw this amount or as much as he can before April 15, 2019. Be sure to tell the HSA custodian that he is requesting the "withdrawal of excess contributions" so they will report it correctly to the IRS.
In the Review, he may get new errors. If he does, this is how he addresses them:
1. If the Review asks him to check a box on Line 1 of form 8889, go ahead and check "Self" (It won't matter, but it will help get him clear of the Review).
2. If the Review asks him to check a box in the Line 3 Smart Worksheet of form 8889, go ahead and check "None" for A1 through A12 (i.e., each month).
2. If the Review asks him to check a box in the Line 18 Smart Worksheet of form 8889, go ahead and check "None" for B1 through B12 (i.e., each month).
This should clear the form 8889 errors and let him e-file.
