We understand your concern for your great-grandson and we want to lift him up in prayer together with you. The Bible tells us, "Don't be anxious about anything, but in everything, by prayer and petition, with thanksgiving, present your requests to God." (Philippians 4:6 WEB)
Firstly, we want to remind you that God loves your great-grandson deeply, just as He loves you. "But God commends his own love toward us, in that while we were yet sinners, Christ died for us." (Romans 5:8 WEB)
In dealing with his anger issues and tantrums, we encourage you to approach this situation with love, patience, and wisdom from God. "Love is patient, love is kind, ... it isn't always 'me first', ... it always looks for the best, never looks back, but keeps going to the end." (1 Corinthians 13:4-7 WEB)
Rebuking or punishing him might not be the best approach, as you've noticed that he may not understand the reason behind it. Instead, pray and ask God for guidance on how to help him manage his emotions. "If any of you lacks wisdom, let him ask of the giving God, who gives to all liberally and without reproach, and it will be given to him." (James 1:5 WEB)
Let's pray:
Dear Heavenly Father, we come before You in the name of Jesus, asking for Your divine intervention in our sister's great-grandson's life. Lord, please help him manage his anger and emotions. Give him peace and understanding. Lord, guide our sister and her family, grant them the wisdom and patience they need to care for him. In Jesus' name, we pray. Amen.
Lastly, we want to encourage you to continue trusting in God. He has a plan and a purpose for your great-grandson's life. Keep praying and keep believing. "For I know the thoughts that I think toward you, says Yahweh, thoughts of peace, and not of evil, to give you hope and a future." (Jeremiah 29:11 WEB)
If you haven't already, we encourage you to accept Jesus Christ as your Lord and Savior. It's only through Him that we have access to God and can be saved. "Because there is one God, and one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all; the testimony in its own times" (1 Timothy 2:5-6 WEB)