Firstly healing the callouses.
Soak hands in warm water.
Soap vigorously when the skin is softened and warmed and use a pumice stone to rub over the surface of the callouses to reduce their height.
Dry hands carefully and apply hand cream to soften skin.
Do this every night.
Sleep with cotton gloves on to keep the cream on his hands. A lanolin based cream is good as it’s heavy and greasy. It should absorb over night.
Over time, the callouses reduce in depth and if he gets new blisters they are not so deep in the epidermis.
He needs to learn how to tape his hands properly to protect the cut skin and give cushioning so he doesn’t aggravate them while rowing and allows the healing to continue.
We wrote an ebook about this – Blisters and Skin Injuries in Rowing
Thirdly – hygiene
His hands will get more resistant to blisters the more he rows. Develop good hand hygiene habits.
Resist the urge to use teeth to pull off skin that’s ragged.
Cut off loose skin and hang nails or burst blisters with sharp nail scissors.
Be aware of water-bourne diseases and wash hands after rowing every day and before eating and make sure his tetanus injections are up to date.