Tyreek Hill is adamantly denying he's ever abused his 3-year-old son ... and says his fiancee is the one who's gotten physical with the boy in the past -- and he has text messages to prove it.
It's all in a letter reportedly sent to the NFL on Thursday by Hill's attorneys, who say Hill has NEVER "punched" his son and was in no way responsible for the boy's recent broken arm.
As we previously reported ... Hill's fiancee, Crystal Espinal, accused the Chiefs superstar of abusing their son in leaked audio tapes .