My brother always realised my mum didn't love him as much as her other children. He has had severe psychological his whole life partly attributable to this fact, and he is now in his 40s.
This. Sorry, but saying... "oh, I'm so sorry OP, that has to be horrible" doesn't cut it for me either.
First of all, "he is difficult and unusual looking"? Well, I wonder why he may be difficult, children are perceptive.
What else? This isn't because of any quality of his. This is you. You don't just love him "somewhat less"/have a favourite child, you actually don't love him like your son at all, but like a nephew.
This is horrible and I fear it could harm all of your children.
Please get therapy asap.
And if you can't "fix this"... Gosh, I don't know the answer. I wish there was an easy one. Or maybe not even an easy one, but a "good one".
But I genuinely believe that all 3 of your children deserve to be loved and deserve loved and cherished siblings. I genuinely think you should love him for his own sake but also for the sake of his siblings...
And I believe that you need to do your best to fix this. But feeling sad and guilty helps nobody, least of all your son...
I'm not sure what to say. Is it a taboo subject? yes. Because I firmly believe that children need love... and I hope you're able to give this love to all of your children.