It's a good idea to repot your plants every few years to encourage growth. To do this, remove the plant from its pot, (use some newspaper if it's prickly) gently remove the old compost or at least break up the rootball. Remove any dead or rotting roots then leave to dry for a couple of days. Choose a new pot only slightly larger than the old one, its important not to ‘overpot’. 

Place the plant in the middle of the pot and fill around with fresh compost ( cactus mix) to the same place on the plant as before, gently press in place. Leave for a few days so he gets used to his new home then water cautiously until the roots have had a chance to grow into the new compost.