I'm so glad you got so many responses when you needed it.  I hope your son is feeling better.  It is very scarey to deal with the vomitting and fevers, especially when so many people don't understand.  I learned the hard way to give an injection right away at home.  Twice I tried handling vomitting with doubled or tripled doses, only to rush to the ER with our own injection.  My daughter would end up all day or night in the ER on IV.  Last spring when my daughter was 3, she had a stomach virus with fever and vomitting.  She vomitted twice in a few minutes.  I calmly prepared the injection and told my daughter to be brave and hold still, and she will feel better.  She perked up within minutes, and never needed the ER, and vomited only once after that.  But I also called the ped and endo, in case things got worse.  For days my daughter told everyone she saw about taking a shot in her bed!  She's needed two injections since then, one I gave and one was given in the hospital during a blood test (LHRH stim test for precocious puberty).  I was so afraid to give that first injection, once I did it, I realized it was not as bad as I had anticipated!  And I saw how it made her feel better!  I still dread this season of viruses, so far so good.  Good luck to you!  And you did the right thing!