UK born child-Tier 2 dependent supporting doc.Manci help plz

Hi everyone!
My son was born in Ūk and he has never been out of Ul.now my wife and my son wants to visit their home country.I have got Tier 2 general visa while my wife is my dependent on Tier 2.we have to apply for our son's dependent tier 2 visa...I have checked on home office website about the supporting documents... it not clearly mentioned any thing... can experts please help me what are the supporting documents I need for my son's visa application
Also if his visa is refused (God forbid) will he get any right of appeal or we have to apply again? will he be illegal in that case?
If he gets a visa and goes out of the Uk n comes backs before his expiry date and then if he stays here beyond his visa date... will he be illegal???? or if he gets a visa and doesn't travel at all out of the uk n after expiry of his visa, will he be illegal??? please manic and other experts please help
I have his birth certificate original,90 days maintenance funds, his original passport, attested copies of passports+BRPs of me and my wife, ready... what else we need please help thanks