It does no good for me to comment on the performance of your son's attorney from a distance. Your son's version of the poor job the attorney is doing may be accurate or a misunderstanding and for purposes of your question, it does not matter. The part of your question that can be answered is your comment that your "son can't interview other attorneys himself being incarcerated." The short answer is ... yes, he can.
You do not say whether your son has a court appointed attorney or public defender in which case, getting new counsel involves other complications but please understand that your son may speak with another attorney at any time, whether incarcerated or not. If you or your son speak to another attorney and that attorney agrees to go to the prison and speak with your son about his case and the possibility of being hired to replace the current counsel, he may do so. He may see your son, speak with your son and discuss his case with him. No one can prevent that. It is done all the time. When wfacing life changing decisions and circumstances, it is important that your son and his attorney have confidence in their relationship and that your son understands what is happening in his case. Remember, your son is in charge of major decision making in his case, including who represents him if he is paying for the attorney.
Whether the current attorney is going a good job or not is impossible for me to comment on. However, there your son is certainly not satisfied with his current representation which is troubling. Consequently, other attorneys must be consulted. If he is incarcerated, then the family will have to do some of the leg work for him. At least once a month I get calls from Moms, Dads, Brothers, etc. who explain the situation. I then go to the jail and interview the client. I see no reason why this can't be done in this situation. The current attorney may have a very good reason for taking this approach. However, if the client is not satisfied, the client must remedy this problem. Talk to other lawyers.
The current attorney and his client may have good reasons for plea bargaining that they are not sharing with you for any number of reasons. If you son wants a new attorney, call one up and have you son see him/her.
I see many prospective clients who are already represented and some retain me and some don't.