Levi Johnston believes Bristol Palin is a TERRIBLE PARENT -- and the proof is in her reality show ... in which she lets their son Tripp run wild, talk back, curse, and show no respect to his elders ... so he's decided to take action ... and seek full custody.

In case you're like most of the country, and haven't been watching Bristol's new reality show "Life's a Tripp" -- she and Levi's son has been a little ball of trouble ... and in one recent episode, even called his aunt Willow the f-word.

During the same episode, Bristol admits, "I'm doing a terrible job disciplining Tripp."

Now, Levi's had enough -- telling TMZ, he's been watching the show and is "disgusted" by his son's language and behavior. Levi says his son is on a "downward spiral" and getting "no real parenting."

Levi tells us, Tripp "deserves a better family" than what Bristol and the rest of the Palins are providing -- adding, "I love my son more than anything ... and I will do whatever it takes to make sure he is raised the right way."

To ensure that happens, Levi dropped by the courthouse in Wasilla, Alaska this week to pick up all the necessary paperwork to obtain full custody of Tripp -- and he plans to file it ASAP.

Calls to Bristol were not returned.