It is normal and only natural for any mother to be concerned about her son no matter how old he may be. However, he is an adult and must take responsibility for his actions and be held accountable. You simply need to provide as much encouraging and supportive words especially when he is at his low points. There is not much more that you can do. You can not change another person. Just keep in contact with his aunt and inquire about your son's overall well being. When your son is in the mood to communicate, you can tell him how you feel about certain things regarding his life. However, your son seems very fragile and your words must be non threatening.
I hope that this has helped. If so, please ACCEPT so that I may be paid for my efforts. If you need more assistance, please let me know. Have a good evening.