5 year old step-son has fever, projectile vomit, heart murmur (diagnosed this hospital stay), and now a rash what could it be?

Gastroenteritis. Most likely a viral gastroenteritis you need to take him to the hospital.
Talk to his docs. Much as I would love to start guessing (guessing diagnoses based upon a very few symptoms is a game doctors play), a forum like this is not the place to get answers to this question. Instead, ask his doctors. They should share with you what they are thinking. If they do not explain clearly, politely let them know that you do not understand and ask again.
Frver, rash ht murmu. The combination of signs & symptoms your son has can be caused by several coditions, since he is aleady in the hospital, I'm sure the doctors are already doing all tests that can help reach a diagnosis. I hope your son feels better soon. Talk to doctor in hospital may be he or she can give you an idea. Good luck. .