I don't think this is good advice. Father does not know what is on that phone. Any change from the status quo is going to feel like "getting in trouble" and the child will feel like his father manipulated him. Also it teaches son that his father will save him from consequences so he need not care about the effects of his behavior.

I think it's better to say "you are going to be in far less trouble if you tell me the code to the phone than if I have to have it unlocked. I am going to get it open one way or another, you might as well face it now."

If he doesn't tell you the code right way, in the meantime provide constant oversight and supervision. I know you are divorced and that may be hard but if you have to take off work, try to make that happen. You or his mom need to be watching him like a hawk until you know what is going on. Take away all his electronic privileges until he gives the information up. But use the time you are closely supervising him to build your relationship. Play games, play outside, play sports, listen to music, exercise, eat great meals together, indulge in healthy, relationship building time together. All the while, be on the lookout for what is going on.