my step son is driving me insane HELP!

So my step son is 3 years old and lives with us full time, with my 2 older boys 7 and 8. When I met my fiance he was living in Texas with his mom and my fiance paid his mom money every week for him. well last June we drove out and picked him up for a visit and once we got back home with him his mother calls and asks us if we want to keep him for a year to get to know us. Now keep in mind when we picked him up he was dressed in raggedy clothes and still drinking out of a bottle. So we agree to keep him out of concern, we had been sending her money how come the bby is dressed so bad and shes dressed so nice, and come to find out his mom lost her apartment cause she wanted to party and have fun. She doesn't send us money, she only calls once a week. Now here's why he drives me crazy. This kid is the most nightmare toddler I have ever met....his mom passes him around to babysitters who gave him whatever he wanted to shut him up. He doesn't eat anything but nuggets and corn dogs and won't try anything. He's constantly crying all day long. My fiance spoils him...we never fought until this kid came around...any advise..? someone help I'm 4 weeks pregnant and this kids driving me nuts.