It sounds like the bird needs to have a nail trim by an Avian Vet or other professional. Some bird shops have a person who is experienced in trimming nails. The macaw is probably also uncomfortable if the nails are too long. Ideally you should find someone who will use a dremel tool to grind the nails, which will shorten them as well as keep them from being so sharp. Another option is to purchase a grooming perch. It is a perch made out of a cement type material and as long as you purchase the correct size, it will help keep the nails blunt. A grooming perch works best after the nails have been cut, however. So a nail trim is still in order. Also, a grooming perch should never be the highest perch in the cage. You should place it where the bird will use it to step to another perch, but you do not want it to be his main perch or sleeping perch. It can cause foot issues if they sit on one of these all the time. You do not want the type perch that has sandpaper or any type of grit attached. A grooming perch will feel bumpy, but it should not have any grit or sand that can come off. It will be a solid material that is somewhat like a rock.
Aside from this, your brother can try to train the bird to step up on a stick instead of his arm. Some macaws hold on too tight and can even cause cuts or bruises with dull nails. Here is a link that can help with stick training. https://lafeber.com/pet-birds/teaching-your-bird/#Stick_Training_Your_Bird
Thank you for asking Lafeber,