If he’d play atom Lego by setting them on fire, immersing them in acid, or using some alternative method to rearrange their atoms, he’d be doing chemistry. If he’d play nucleon Legos by rearranging their neutrons and protons into different kinds of atoms, he’d be doing nuclear physics. If he’d smash his pieces together near the speed of light to rearrange the energy, momentum, charge, etc., of their neutrons, protons and electrons into new particles, he’d be doing particle physics.
