I am just handling it by lavishing as much attention as I can on Aidan, I have to be careful though that the other kids don't feel neglected as a result. I made Aidan some chocolate milk and he was sitting at the table drinking it when the others came home, they made a huge fuss, although neither of them are too keen on chocolate milk. It was a case of one child having something they didn't have.
I try to treat them fairly, and if I buy them new clothes I try to get something for Aidan as well, but then my son will make a fuss - in front of the kids - and says "I buy Aidan's clothes, you don't" and then he will add "he can have the clothes that Conner has grown out of" (Conner is 7). My husband and I have official custody of the other two and my son has custody of Aidan, and he makes sure everybody knows that. It is an awkward situation, I think Aidan must feel singled out.
It's difficult when we teach Aidan one set of values and then my son comes along and tells him something different (e.g. Aidan might complain "Conner hit me" - or took my toy, or something - and I will talk to Conner about it and try to resolve it with words and my son will say,something like, "well stand up for yourself, hit him in the head".
At least we took one step to make him feel included in the wider family. When Andy (son) moved in, we converted the garage into two rooms - one for him and one for Aidan. When Andy went back to work we shifted Aidan in with the rest of the family and now he shares Samantha's room (coz she has the biggest bedroom) and I shifted his set of drawers with all his clothes in up to the lounge - Andy objected loudly but I told him "look, it is too much for me to go and get clothes out of the garage for him all the time". He accepted that, and it doesn't make the lounge any more messy than it was anyway.
The biggest difference between the way he is treated compared to the other kids is that he sees his mother all the time. I am very strict with the other kids and they are only allowed to see her for a few hours every three weeks or so, under strict supervision, but Andy is always dropping off Aidan at his mothers and I never know until I go looking for him. When I object "Andy says "it is because I knew you'd make a fuss".
We have four children really, aged from four to 30. Andy is the 30 year old but he basically just one of them.