@grievingDad,
I truly do not intend to be callous in this situation, but I will make a few points here and some may be helpful.
1) I don't know who you are, and since I do not know you, I cannot be certain of your motives for even trying to retrieve deleted messages. If you have truly lost someone and your motives are as simple as they appear in your firt message, my heart goes out to you and I wish you all the best in trying to retrieve your son's messages. I only say this because I would not even attempt to retrieve deleted messages for someone if they were doing it for spiteful or illegal reasons (Example: Jealous lover, disgruntled employee, etc.). There are probably more illegitimate reasons to retrieve another person's messages than legitmate ones. I am guessing that is why you will have a difficult time finding someone to help you with this directly. I am not an attorney, but even helping you to retrieve deleted messages sounds like a slippery slope.
2) You may be able to get an attorney to petition AT&T for a record of the messages. I believe that they have to keep them for a certain period of time for legal reasons. I'm not sure how accessible the messages will be. I remember trying to get a copy of one of my own messages (many years ago), and I wasn't able to get it from them. I know that messages are subpoenaed in court, but you might have to involve a judge to get them.
3) If your son used Hangouts for messaging (not the stock messaging app), you may be able to get the messages through his Google account. You might be able to ask Google for his password. I suspect that it would invole contacting Google and send them an official copy of the death certificate. I doubt that it would be easy.
4) It is possible that you could something wrong and brick the phone. In other words, you would never get anything off of it.
5) Consider the possibility that the messages may really be gone forever. Some things in the digital world offer the ability of recovering from "Oops!", but some things are meant to be deleted permanently. I am not trying to be a pessimist. All I am trying to do is counter the notion that everything is hackable and data is never really lost.