None. Macaws are NUTS enough as it is! ... Sorry, that was full of fail, lol
You could probably give more high-fat nuts to your underweight lovely, and when he catches up just keep them both on the less fatty nuts. I give my B&G a varying amount, since it depends what else I'm giving him that day. Recently he's back to only liking pistachios, so he gets about 8 a day max (he drops a lot of it anyway, I don't know why I bother hehe

)