My husband intends to marry a second wife

This week’s topic:

I am 26 and married although through a customary marriage. My husband has paid dowry and we have two daughters. My husband has been seeing another woman for a while and she bore him a son. This has caused me so much trouble that I am in and out of hospital from stomach ulcers. He wants to marry her so he is suggesting that he pays dowry for her to marry her customarily then he can formalise our wedding in church but with her remaining as second wife. I am not sure if and how this can work but I am also afraid that he gives her too much attention often leaving us for days only to come home flat broke. This is stressing me so much, please advise...