He was only in their care until age of 18, IF he was in the foster care system. Its said he wasn't in the system but the Barkers were his legal guardians because they knew his parents and if they didn't, he would be in the system - I imagine they knew his mum as his dad passed away when he was young. You have to realise, he was NOT adopted by them legally.
I'm not justifying them giving him as little notice as they did but clearly they've helped him out hugely in life if what all the rumours around his parental situation are true and clearly he holds no bad feelings. He has still been around since at Saffron's party and Casey seems close to him too as he's mentioned him once or twice. They took him on holidays (Egypt!) with them and if the last xmas he had with them is any indication, was spoiled at xmas along with their actual kids. I imagine they told him to leave because at 18, he probably came in to an inheritance too and he was doing well for himself. Obviously its speculation of inheritance but it seems he is definitely well off and I cant imagine music on his level is paying well enough that he's on holidays, buying designer clothes, renting in LDN and paying for studio time, photoshoots, etc.