CADad,
I'm not sure what the Pierce College website is saying.
However, the 40/60/80 rule(14.4.3.2) does apply to transfers from 2 or 4 year colleges. (see 14.4.3.2.2)
In the OP, the player is in very good shape for the 40/60/80 rule, because the NCAA doesn't speak of "juniors"; instead it refers to the 3rd year of collegiate enrollment. The OP was still in high school while amassing the dual credits. He's ahead of the game.
In your son's case, transfer eligibility wouldn't be a problem if he gets in a semester with 12 hours of transferrable credit. Will baseball units transfer?
However, besides meeting the transfer eligibility requirements, he also needs to meet academic progress. He needs to make sure that he'll be in compliance with the 40 percent.