BIG HUGS
I am ever so sorry for our horrid time!
It sounds like your little boy could have gone into acute heart failure, which can happen at any time of life if a piggy has inherited a genetic timb bomb. Injecting steroids does unfortunately not work with guinea pigs; it slows down the organs instead of picking them up. Many general vets are unfortunately not aware of that but it would not have made a huge difference in the end - once the heart goes into failure and the body is closing down, there is nothing you or a vet can do.

You have done all the right things, so please do not feel guilty or like you are failure; nothing is your fault, it's just cosmic bad luck!
Here are our tips on what you can do for the bereaved companion right now and in the medium term. Ideally you find him a companion of his choice at a good rescue that rehomes only quarantined/vet cared piggies that are fully healthy at the time of the adoption as well as properly sexed so you can minimise the risk of any unpleasant experiences. At this age, babies are desperate for company. The listed rescues will also be there if your bond runs into major trouble during the life time of their adopted piggy or if you can no longer keep your guinea pigs.
Looking After A Bereaved Guinea Pig (includes lists of recommended vetted good standard rescues in various countries that we can vouch for you being in safe and experienced hands)
Our human grieving guide also contains resources of how to deal with a pet death for children:
Human Bereavement - Grieving, coping tips and support links for guinea pig owners and their children
You may also find our new owners guide collection helpful:
Getting Started - New Owners' Most Helpful Guides