Hi, your best bet is looking at
http://www.starlingtalk.com/babystarlings.htm
that site deals with pet starlings (young and adult) and will give diet and care details. One thing, stop feeding it on earthworms, as they are not part of its natural diet and can be toxic.
Hope it all goes well for you, just be aware that young birds can do downhill very quickly and for no apparent reason, so just prepare yourself for him not pulling through, despite all your best efforts. Good luck!