"I don't understand this part here."
"Well, he's thinking back to a few months ago when he had this dream."
Rewrite.

"I don't understand this part here."
"Well, he's thinking back to a few months ago when he had this dream about this thing
that happened when he was sixteen."
Rewrite.

"I don't understand this part here."
"Well, he's thinking back to a few months ago when he had this dream about this thing that happened
when he was sixteen that made him remember this event from his childhood."

Aha!