Joyce goes to visit Will in his fort in the woods and later, after he disappears, she and Jonathan go searching for him there in episode 1x01 “Chapter One: The Vanishing of Will Beyers”. Steve gives Dustin advice while they leave a trail of meat to lure the Demodogs in episode 2x06 “Chapter Six: The Spy”. Dustin & Lucas get into a fight while walking down the tracks with Steve & Max in episode 2x08 “Chapter Eight: The Mind Flayer”.
Appears in 3 additional episodes.
Also appears in “Chapter Three: Holly, Jolly”, “Chapter Five: The Flea and the Acrobat”, & “Chapter Six: The Monster”.
Source: Stranger Things — Wikia