Sex drive

My husband is a nympho. We have 3 kids, the two girls are his from a past relationship and the boy is mine from a past relationship. We got together When I was 6 months pregnant with my son. At that time I was a nympho also but since I had my son (feb 6, 2011) my sex drive has gone way down. I'm never in the mood anymore. There are time where I will have sex with my husband just so he doesn't get upset. How can bring my sex drive back up? I'm scared that if I don't do this he will find someone else. Please help!!!!!!!!!