Just like his old man.
In these lines Heaney is putting his father and grandfather on a higher pedestal. He notices how hard his father strains to do his work. He also views how persistent his father is as he drinks some milk and right away gets back to digging. Heaney uses his father as an example of a good work ethic, wanting to be a hard worker also. Even though he does not use a spade, Heaney implements the same digging mentality with his writing.