Michael Lohan -- I'm Filing For Full Custody!!!

What does it say about Kate Major's condition that Michael Lohan thinks she is so bad off ... he needs to file for full custody of their baby?

That's right ... Papa Lohan tells TMZ he fears so much for his young son's safety ... he wants a judge to make him the primary parent ... this following Major's arrest Thursday night for DUI and battery.

According to our sources, Lohan will go to court first thing Monday morning to file legal docs. We're told he wants the court to give Major monitored supervision ... that is, once she gets out of jail.

Lohan tells TMZ he thinks Major's doctors are to blame. He says she's only gone off the deep end recently -- since they put her back on meds -- and he wants the docs investigated.