It seems like your husband was never Christian and did not know Jesus or God personally. Rather he is self centered and probably was biblioter and using scripture to his own convenience. If he knew Jesus, he would never denounce him or the faith which is a Christ centered cross based faith. To follow Islam??? that is a strict law based religion which means he has to do much more dedication for atonement and sanctification. If he couldn't make it as a simple Christian, he is doomed.
In any event, it would appear he is choosing Islam in order to have a second wife, one who could give him a legacy I presume. I don't think it would be healthy for you to attempt to follow him down that path, because Christianity, as well as the general world ideal, of one man one woman in a monogamous relationship, in the covenant of marriage as God has defined it to be. It is in this covenant relationship that his image and ideal for us is displayed and intimacy of a man and woman can best be found and sought after inside of the covenant relationship of a man and a woman, singular.
Tell him you won't stand for it, and if he chooses to convert to islam and marry another woman you want a divorce first.