There is nothing to stop you marrying him, whether he divorces his wife or not. Your marriage to him would not be counted as wrongdoing towards his first wife, because plural marriage is something that is regarded as commendable according to sharee’ah for the one who is able to treat his wives fairly. With regard to the problems between him and his first wife, and his thinking of leaving her, you have nothing to do with that, and you will not be sinning, so long as you do not ask him to divorce her or encourage him to divorce her in any way.
But if he does not divorce her and he wants to marry you, then he must treat you both fairly. If he fears that he will not be able to treat you fairly then it is not permissible for him to engage in a plural marriage, because Allaah says (interpretation of the meaning):
“… then marry (other) women of your choice, two or three, or four; but if you fear that you shall not be able to deal justly (with them), then only one or (slaves) that your right hands possess”
Sheikh Muhammed Salih Al-Munajjid