I honestly don't understand at all why you have a problem with people saying they would consider their existing children and how they'd cope with it before adopting. Honest to god what is so terrible about that? Don't most people consider their children before doing something that changes their child's life so hugely?
You're acting as if people don't ever consider the impact of further biological children either, I appreciate some don't, but a lot will. I certainly thought of our older children before TTC.
Comparing that to leaving a baby at the hospital is ridiculous and complete hyperbole. The only comparison would be adopting a child and then giving up when you experience any issues. Obviously that is not what you want... Hence the need to consider it and the other members of your family before you do it.
But anyway, that's not OPs situation. I jsut find it mad that people actually think it's offensive to say you should consider your other children first 
In OPs situation, I can actually see how a young adult, university age so could be what, 18, 19? May possibly be upset at their Dad living with another child (conceived or adopted!). He could be the best Dad ever on his time but the fact is, not living with one of your parents can be upsetting no matter how great they are and so I can see how watching your Dad, that you didn't get to live with, playing happy families with his wife and further children could be upsetting. Yes, even as a young adult...
Not to say he shouldn't adopt the little boy but I don't think it's hard to understand why it might be difficult for his son to see.