US is a free country. You can do whatever you want if not against the laws. You can leave your current employer even just after he wrote a letter for you. The thing you should worry about is that you get another RFE and still need his help. At that time, you will be bustered. So I suggest you stay with your current employer till you get your GC approved. Usually, you will get a result for your GC after the RFE very soon.