Nathan Griffith -- Jenelle's 2nd baby daddy -- says the "Teen Mom 2" star is purposely keeping their son away from him ... and it's driving Dad crazy.

Nathan just wants to spend time with their 4-year-old son, Kaiser, but he claims Jenelle is using phony excuses to stop Nathan from seeing his boy.

He says Jenelle has been giving him the runaround ever since she was cleared of criminal wrongdoing in a child abuse investigation -- which was triggered by Nathan going to police.

Nathan says he doesn't even know where Kaiser is on any given day, 'cause Jenelle doesn't keep him informed ... and he's just about had it with her antics.

Talk about baby mama drama.

As for Jenelle's other ex ... Courtland Keith Rogers was busted over the weekend in North Carolina and accused of assaulting a woman.