If he didn't know this was a strike offense, and his PD will back him up on that he can ask the court to change his plea. Of course then he'll be back where he started, accused of a crime that bears a 3rd strike. But then at least you'll be in a better negotiating position.
Make sure the attorney checks and verifies that he does indeed have a strike. You wouldn't believe how many times the DA gets it wrong because of errors in RAP sheets.
If you have a lawyer then you should ask them. They would certainly, i would think, know more about your son's situation than we will ever know on an electronic forum. Good Luck!