Our two (eldest a girl and youngest a boy) never had any issues and are still best friends at 17 and 14 respectively so I can't advise from their perspective but I can give you an insight from my own.
My brother came along when I was six and a half. From being an only child I now had an interloper who would go on to break my toys and water down any attention I may have had. As a result, I gave him quite a hard time. We've never been very close as a consequence and rarely speak, although we get o better since my father dies (that's another story!). The conclusion I've come to from analysing my experience is that my parents didn't handle it very well. So my limited advice for what it's worth is simple. Don't take sides. Be firm about squabbles but be neutral. Make sure you give the older one a bit more affection and reassurance in the next year or so as he's still very young and will need time to adjust but at the same time make it clear that negative behaviour towards his brother is unacceptable.
Sorry, no advice but I'm glad it's not just my DS. It's really upsetting isn't it? Of course, no one else ever admits this in RL. It's all "oh yes, they LOVE LOVE their new sibling". There is indifference in our house too, on one side, and adoration on the other.
My 3 year old son is a very loving child. But he simply cannot tolerate his younger brother who is 16 months. When youngest was born my eldest basically ignored his existence up until youngest could walk and follow him round. Then eldest gets really angry with youngest if he goes near toys he's playing with he never shares with him unless we make him. If I leave them in a room on thier own it usually results in youngest crying due to being pushed or smacked by eldest. Don't get me wrong there have been break through days where eldest has cuddled youngest or had a quick play with him but I'm wondering if there is any way I can improve thier bond. As youngest adores his big brother. We have another little boy on the way in Feb it will be very interesting to see if this has any effect on thier relationship or how they will take to baby.