In this anime a young boy tries to feed a robot lying on the ground with a big rock. I can only remember this scene and this is the only scene that I can 100% sure it happened.
Tried to search online with " young boy call robot with a flute ", after searching for years, and latest (today), quite a lot results show Yamato Takeru, which is totally not the one.
100% remember/accurate: he tried to talk to the robot but no response, he climbs onto the robot and feeds him a rock, the rock is as big as himself (the boy), he puts it into the robot's mouth and anticipate.
79 to 99% accurate: the robot did chew on the rock, they are resting on a mountain rocky area.
65% accurate: this robot will only move when someone plays the flute sound/music, or only by this young boy. Move by walking, not summon or others.
The art might be 80s or 90s
Can't remember whether watching on TV or renting VHS tape cassette
young boy should be below 12 y.o.