I'm so very sorry for your loss.
When our twins were delivered at 19 weeks after their hearts had stopped, I was given a tablet to stop milk production. You absolutely should have been offered this.
In terms of post mortems, we decided to allow them for our twins because we wanted/needed to know why they died if it was at all possible to find out. We were called to the hospital to discuss the results and they had concluded that our babies died due to placental abruption.
We also opted for the funeral offered by the hospital. We were told at that time (January 2019) that we had to have a funeral by law due to the babies' gestation. We were told we could attend if we wanted to but didn't have to. We went, of course, to say a final goodbye to our little ones. With only myself and DP plus my parents there, it felt like not many people cared if I'm honest. I was sad for them and felt they deserved more. We chose the cremation, requesting that the twins be kept together at all times. Their ashes were returned to us a couple of days later and we've chosen to keep them at home.
We also have two Aching Arms bears and have donated bears in loving memory of our three babies (the twins and our son who we lost neonatally). They can be a real comfort at the very saddest of times and it's a really lovely thing to have the option to offer a bear to other bereaved parents. You absolutely do not have to give it to your son unless you want to and feel ready to share your bear! There is no rush to make any decisions now.
As for letting people know of your sad loss, again, there's no rush. It took me several weeks to find the words. I explained that we'd said goodbye to our babies who were now at rest. I didn't give any details. I didn't need to.
I'm thinking of you and your family
Look after yourself, you've been through a very traumatic time.