Just a thought, might be totally off mark. Taking him by his word, so, 'I don't want to be a good boy' meaning exactly that… Basically he is refusing to accept your good care. It occurred to me to wonder, why not? and, what DO you want?
And I thought, perhaps he doesn't want to be a good boy (etc.) because it feels to him that this, being a good boy (or being safe, or having hand cream on his sore hands, or anything really that you are offering him) is coming at a price, a cost that he'd rather not have to pay.
So, either what he really wants, is for example, to be back with his foster carers, and he feels that if he accepts all the good care you are giving him, then he is saying no to his foster carers. He'd rather THEY did all that caring stuff, that's what he wants, and if he gets it from you, he can't get it from them, or is being disloyal to them. In a way, I can nearly hear his previous FC saying to him 'you be a good boy for your new parents'; so that is what he is refusing.
Or, he is worried that accepting all the good care you are giving him, will come at a future cost. Perhaps he is worried that at some point he will have to pay you back (in other words, he feels he doesn't deserve your good care just for being himself, but instead will have to earn it retrospectively). Or it may be simply the personal emotional cost he projects for the future, when, inevitably (from his experience), you will let him down just as all other adults have let him down, or will leave him/send him away. So in order to protect himself from that future heartbreak, he refuses now to accept your care, your love, your respect.
It sounds to me as if the underlying emotions my well be a combination of lack of self value, fear of being hurt again, distrust of the reality/longevity of your love and care for him. And/or, loyalty to his previous carers and denial about the realities of him having moved on to you.
I don't really know, but maybe you could try bringing up the topic of his previous carers, and listening carefully for how he feels about them and the move. Then, depending on what you 'hear', try to reassure him that his previous carers still care for him very much (they didn't just send him away lightly) and it is ok to miss them (it isn't disloyal to you), but that equally, his previous carers want to see him happy and cared for (so it is not disloyal to them, to accept care from you). Or, if you 'hear' that this bit is less of an issue but the more general distrust of adults and lack of self worth is more prominent, that is something you can only address with time, and with consistency in all things regarding these things, e.g. unconditionality of your love, the way you discipline, etc.