Did a little looking around. Since it sounds like you want something that would instantly mellow out or sedate your son, I don't think that you will find it. Furthermore, depending on his age - trying to stick an epi pen into someone who is agitated is difficult and possibly dangerous for all (including you) involved.
If you can answer some of the questions we asked, perhaps we can suggest alternate methods to help you.
It would be helpful to know how old your son is and if the meltdowns also occur at school?
I'm dead set against giving children medication to help with behavioral issues so I cannot offer you any advice on the use of an epi pen, except to say don't do it. Try natural alternatives such as fish oil gummies and lots and lots of therapy.
This should be discussed with your doctor ,could you tell us why he has meltdowns and what are his special needs?