First of all, you shall understand you will be a dead meat if you stay in the current group.
When talking to your perspective boss, emphasize two things:
(1) You are determined to join his group, even if it means you will have to go all the way up the management chain and get HR involved. Without assurance from you and seeing your resolve, the new boss may not fight hard for you. Nobody wants to the first "bad" guy.
(2) Bluntly ask him to honestly tell you whether he has any concern. Don't second-guess by yourself. If he mentions any issue based on any negative information from your current boss, you shall clarify as much as you can and label it as the result of the sour feeling of your current boss.
Good luck.
