My son's last name was misspelled on legal documents charging him with 20 yrs

No. The sentenceing cannot be dismissed unless there is some authoritative evidence that it was a misidentification. A misspelling would not be enough. However, a correction may be made in the record to correct the spelling of his name

Typographical errors such as this are rarely sufficient to obtain legal relief in criminal court.

A sentence can be overturned in some situations, the most common of these involve an appeal to a higher court due to an incorrect decision of the lower court in their handling of the case, including incorrect sentencing. However, a mispelling doesn't matter. Sounds like you have concerns over your son's sentence, so I suggest you contact a local criminal defense attorney to discuss any chances your son may have to appeal his sentence. There are so many reasons to overturn a sentence, that avvo won't give me the space to list them! Look around for an "appellate attorney" to discuss your options. Good Luck,