God is not punishing him with the torment of guilt. God does not do that. According to the Bible, guilt does not come from God, it comes from this world (2 Corinthians 7:10). Do not blame God for things He has not done.
You should be thankful to God for saving your grandson's life. Did you really want him to die? That is what you imply. Wow. I am sure if your grandson had died, that you would blame God for that too.
Unfortunately, you do not know God. God is loving, forgiving, generous, a million times greater than you, a million times smarter than you, and always does what is right. How dare you blame Him. Someday you will stand before Him and then you will know. He will look at your life and see if it was one lived for Him and according to what He demands.
How do you know that the three who lost their lives are now in a better place without torment? Unless they trusted Jesus, they are in a place that has much more torment forever and ever. Unless you change your ways and trust Jesus and what He has done, you also will end up in the same place of torment. This does not come from me, it comes from the Bible (it is mentioned there many times). The road to heaven is a narrow road that few find.
It amazes me how people can ignore and disrespect God (a life that does not treat God as God is disrespecting Him) their entire life and then when something goes wrong, blame Him. If you do not treat God as God and trust Him, do not blame Him for what you have messed up.
PTSD is curable and is something that people get over. I know many people who have conquered it. Give your grandson some time, give him some love, and make sure he is getting professional help.
Whatever you do, do not blame God. Ask Him for help and He will help. Trust Him and He will take care of it.