There is something wrong there...I could see it with a six year old, as a "I'm not the baby anymore" thing, not an 18 year old. How about trying to find a counselor to talk to him? Or is there maybe a pastor or coach who might be willing to help? Sounds like he's got something serious going on.

I do not know many skids who just absolutely want new half siblings. I wasn't enthused about my half sibs at first, either. He'll get over it. He won't have a choice.
I can understand him not wanting the new baby using his blanket. I didn't want "the new kids" touching my stuff, either.

I think it is jealousy IMHO. My ss was jealous of my grand baby. My grand baby was 1 year old when I married my dh. My ss was 12. He did things that to me showed dislike or jealousy up until he grew up and moved out.

Examples: 1)Took a pillow from grand baby. (ss14 at the time) The pillow is one that I had purchased after getting married to dh but my granddaughter loved it . 2) ss(16 at the time) tore up a toy I had gotten for granddaughter. It was made out of soft foam & I put it on the kitchen table & ss tore it apart. 3) Granddaughters turtle. I live out in the country and found a turtle & gave to granddaughter. We put a little paint on the turtles back and let it go in the yard. Being out in the country we see turtles all the time and thought it would be fun to be able to identify this turtle if we saw him again. I then see ss17 at the time putting the turtle over the fence.

Anyway, I say jealousy is the root behind this behavior.

I think it would be weird to have a new baby sibling at 18 alone without throwing in that it's with a different mom. When he is ready to face adulthood and possibly have his own kids, he has a baby brother? Coupled with the fact that teen boys often don't know how to deal with a baby (grown men don't often either) I wouldn't expect him to bond until it's walking, talking, or able to throw a football. Don't expect them to have too much of a relationship with that age gap. As far as the blanket, I don't think it's terribly serious, but you won't know unless someone asks him about it.

Echo...how sad your son had to go through such a terrible experience. I think many people would be surprised if they realzed how many people have some sort of security item.
My youngest brother who is in his 40's has had a small swatch of material from his baby security blanket that he never let go of. Approximately 8 years ago, when civilian transporters over in Iraq were being held hostage and gruesomely murdered. That is where my brother was working for the year. He came home safe, but it was later that he told me that being able to put his hand in his pocket and touch the small swatch of material was a comfort to him, when he needed that.

It is not something he ever mentions, for him it is private. Maybe like the way some people will clutch a religious medal in times of stress for comfort.

I had a baby blanket that I loved throughout my childhood. I didn't take it anywhere with me, but at home I always slept with it. I didn't take it to friend's houses or anywhere I slept other than home. I was a bit embarrassed that I still had it as a teen, but since I only kept it in my room, it was more of a secret for me. When I was in college, I obviously left it home. One weekend my freshman year, my sister-in-law and I were at my parents' house and re-arranging my bedroom there. She saw the blanket and made me throw it out. I really looked up to her (she's 7 years older than I am) and I wanted to avoid embarrassment, so I did. I will always regret having thrown it out. I told my therapist about it more recently, and she was so saddened by that. She also believes it's a good thing to have a security blanket from childhood, and that it's something to keep for life, or pass on to your own children. I don't hold it agains my SIL, but I wish I had said I will just put it in the closet.

My oldest son (3, almost 4) has a security blanket that he loves. I will never take that away from him, and hope it will provide him comfort for years, or life if need be.

I feel bad for the boy here who cannot accept his baby half-brother. I think his dad should talk to him and try to reassure him in some way. And definitely not discuss the baby blanket.

18 is a terrifying time when you consider all the changes that rapidly come in short succession. My own half siblings were born at 18 and 20. Please do not minimize the impact the birth has on him. His father needs to reach out.

Both of my kids had/have stuffed animals that were comfort items. Both kids took them to college. People who make fun of such are the ones with an issue.