I am engaged, wedding in a couple of months. Fiance and I have known each other for 20+ years, but dating 1 year. She has an 18yo son, 9 yo son, and 5 yo daughter. The 18 yo and 5 yo love me and we get along great. Initially the 9 yo loved me too. He's very affectionate and often holds my hand when we are walking or lays close to me on the couch etc. He has had some struggles with anxiety, mostly feeling nervous about being separated from his mom when he goes to school etc. It was really problematic for a few months and couseling wasn't helping so he was started on some anxiety medication and the issue disappeared. He didn't like the side effects and did seem a little distant so his mom and biodad decided to take him off the meds for the summer to see how he did off of them. We got engaged about 4 months ago, right around the time he got off the medication. Ever since he has been saying he hates me and I'm mean and I'm the worst person ever. He says all I do is take away his mom and he never gets to see her. The interesting thing is that he knows I'm not mean, and if we ignore his outbursts, he'll often revert back to being affectionate. But if someone says "see, you do like him" he says he hates me again and isolates himself from me.  When his mom and I are sitting close to each other, holding hands, or kissing, he attempts to break us apart and get between us. It was sort of cute and funny initially, but since he's been saying he hates me and he wishes I was dead, it's become a little pathologic and scary. We are planning to have our own children together and this really sets him off. He says he will throw the baby into the lake and stuff  like that. To add to the problem he says he wants to go live with his dad since he never gets to see his mom and I'm the worst person he's ever met. Neither of us want this of course. 

This seems like some obvious separation anxiety and attention-seeking behavior and we're focusing on making it a point for him to spend extra quality time with his mom, but it is interfering with our relationship and I don't want this to bleed over to the daughter. She so sweet and we have a great relationship. I don't want him to ruin that. 

I'm sure some of you have dealt with similar issues. What has worked? What can I expect?