's son tragically died from overdosing on fentanyl ... TMZ has learned.

The "Pawn Stars" star, through his rep, tells TMZ ... "Yes, I can confirm Adam died from a fentanyl overdose. The fentanyl crisis in this country must be taken more seriously.  It seems it is just flowing over the borders and nothing is being done about it. We must do better."

TMZ broke the story ... the Harrison family found out on Friday that

from a fatal overdose. At the time, the circumstances surrounding the death were unclear and Las Vegas Metro PD told us they were investigating the case.

The Harrison family told us ... "Our family is extremely saddened by the death of Adam. We ask for privacy as we grieve his loss."

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Ricklater posted an emotional tribute to Adam on his IG, saying: "You will always be in my heart! I love you Adam. 💔"

Adam is one of Rick's three kids -- and his second son with his first wife,

Kim. He'd mostly been off the radar in terms of Rick's show and store ... seemingly having chosen not to appear in any of the episodes.

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Because he was never involved in the show, not too much was known about him ... but we heard he worked professionally in some type of trade.

Adam was only 39.