Pat, RPO would certainly help. Macaws in the wild eat tons of Palm fruit including the nut inside.
As you have already described, I would just keep all the food in front of him you have. You know he won't over eat. It takes time to put weight on.
I don't remember if you have taken Harley to a vet yet or not. But, since you mention supplements, I would take him in for a full blood Panel. Starvation can cause several serious health issues that only your vet could diagnose and recommend the best treatment to get him back on track quickly.