Return to the Ashina Reservoir once you reach Ashina Castle and you'll find him above the cell that the Wolf was locked in at the beginning of the game. At this point, if you have started Doujun's questline, you will have the option to send him to the Abandoned Dungeon, doing so will end Jinzaemon's questline but advance Dojun's and you can only choose one per playthrough.
Head over to the Abandoned Dungeon and follow it through to the right to reach the Bottomless Hole Idol. You'll find Jinzaemon in the cave here and can speak to him to listen to all his available dialogue.
Head to the Guardian Ape's Burrow Idol in the Ashina Depths and as you exit the cave, you'll find Jinzaemon to the left, in the Hidden Forest so once again, speak to him until he offers nothing new to say.
Once you reach Mibu Village and eventually make your way to the Water Mill Idol, you'll find Jinzaemon slumped up against the wall as you head up the path. Speak to him once again and listen to everything he has to say before heading up the path to defeat O'Rin of the Water. Return to Jinzaemon following her defeat to see the end of Jinzaemon's questline and obtain Jinza's Jizo Statue.