A 180-day writ is where a defendant who
1) is in the MO Dept of Corrections AND
2) has pending criminal cases in MO
files a document (which is available in any MO correctional facility) demanding that any pending matters that he has elsewhere get wrapped up in 180 days. Then the officials at the prison facility have to take some actions to get notice out to the counties where the untried criminal matters are to let them know that the defendant is requesting that his criminal cases get finished up.
My recollection is that the 180 days starts running when a county or circuit receives the request. If the matter is not brought to trial or otherwise resolved within that time period, the case is dismissed with prejudice (meaning that it cannot be refiled -- it's gone for good).
Now, there are lots of caveats, exceptions, and ways prosecutors can get around this, but that's the general rule. A lot will depend upon the particulars of your son's situation.
My statements are my opinion solely based on the information provided in the question, and that opinion can be inaccurate if the information is inaccurate, or if there are additional important facts that have been left out of that information. Always be careful about including any information in a public forum that could be used against you in any way.
One tactic is to file a speedy trial motion (180 day writ) which does put some pressure on the prosecutor and court in Lincoln County. Another possibility would to hire an attorney to look into your son's case and if it is his choice and in his best interests the attorney could negotiate a concurrent sentence or time served. Contact an attorney.