I am so sorry for all involved
Prayers
On what to say: First what not to say don't talk about all the "should ofs" - because you & I believe they should have went to the ER Vet - it was probably the best chance for survival. But your son is already beating himself up enough I'm sure - and even the ER Vet might not have been able to save this little life
Just reassure your son that you are there for him. He knows this was an accident. Just love him (In time he will be ready for another little one) I can understand why he doesn't want you to come (but as a mom you want to be there to comfort) - but he is probably physically & emotionally exhausted from the last day & watching over this little one. Your son probably wants to go curl up & rest. Give him space. Maybe tomorrow you can make them dinner or something.
Again, I am so so sorry