He needs to make note of it, and make sure that in the divorce decree, it is stated that the child is allowed to have the cell phone. If the divorce were finalized, her keeping a phone from the child that the father has paid for would be against court orders because each parent is to allow the natural developement of the parent / child relationship without interference. If he wanted to try to have the greater custody of the children, this issue could be brought up to show that she is interfering and would not be a good choice for primary gaurdianship.
If she is doing this with the phone, I can almost guaranty that she will try to prevent him from seeing the kids if she gets custody. If she does anything to prevent him from seeing the kids, even if she tells him the kids are sick during his visitation time, he must make a police report each and every time she does this. Start from day one. My husband was "understanding" with all the "plans" his kids had, and did not file reports. Now, his rights have been stripped. Part of the reasoning behind this is that he had not seen his kids. Your son will not be "the bad guy" if he simply tries to enforce his visitation, but he will be a "bad parent" if he does not spend time with them. If he is in California, tell him to fight for full everything. California is very "female" when it comes to this. My husbands ex agreed to shared everything, but later had him stripped.