Similac Formula Causing Vomiting and Diaper Rash

We were advised by our doctor to use Similac formula with a prescription. I've been using it for 3 days, but my baby experiences vomiting after consuming it, and he has also developed diaper rash. The doctor recommended it due to difficulties with weight gain and sleep disturbances. Although there has been some progress in his weight, he continues to be restless and cry incessantly. I'm thinking about completely quitting. Is there anyone available to provide assistance? Has anyone else been facing similar problems lately?