What a difficult situation. I applaud you for taking this boy into your home and your heart. You are suddenly parenting a child who is dealing with both loss of his mother and the changes in expectations for his behavior that came with coming into your home. I’m sure he is struggling. And i’m not at all surprised that you are struggling too.
Kids grieve in many different ways. It is not unusual for anger to be mixed in with the sadness. It is normal for him to feel angry about the loss of his mom, upset that his life has been turned upside down, and unable to manage his many emotions. Please don’t feel that at times you aren’t up to the task.
So, yes. You should go back to therapy, not just for him but for your whole family. A family therapist will help you all (you, your fiance and the boy) understand the changes that are necessary to keep him physically and emotionally safe. Therapy will give him a safe place to express his feelings and to hear your love as well as your concerns about how to help him. A therapist will also give you new tools for providing him with the loving discipline he needs.
There is no way you could have been prepared for this sudden leap into parenthood. Give yourself, your fiance, and the boy the gift of the support and practical help a therapist can offer.
I wish you well.