Buy him something flexible.
A battery powered ride-on toy is exciting for a little while but all you can do is ride on it.
A climbing frame with a den or tent on it plus some planks, lengths of hose, small squares of decking and a camouflage net can be anything he wants them to be.
A castle, a prison, a pirate ship, a desert island, a house, a hobbit hole, a hospital, a teepee, an aeroplane,......
A patch of garden he can make mud pies with will give him opportunities to develop his imagination and his motor skills.
Perhaps a tree house with a rope ladder if you have a place to put it. Things to climb are a definite must!