How do I block him from bringing a second wife?

Dear Harold,

We married under Kikuyu customary law but my husband has, of late, been hinting about marrying a second wife. I am disturbed by his intentions as I know that life will never be the same again. Moreover, how will I ever explain to our young children that their father has another wife and may not be spending some weekends with them? There is more at stake as we have invested in property and the Nairobi Securities Exchange during our 12?year marriage. I have read from this column before that a customary marriage — where dowry is paid — is potentially polygamous. How can I salvage the situation?  Shiro, Kiambu