One thing my boy loved at that age was shopping and cafe, i collected all our old tins, jars, packets, bought a tea set and a til from a charity shop, put some coins in it, used an old purse of mine. We also made a cooker out of a carboard box which had an opening oven with a magnet to close it. we painted it together. then we would make a shop and a cafe and invite teddies. lots of fun you can do together and then they can play alone too.
If he loves books then read to him as much as you can, it might not last long. if he likes to draw then let him get messy with paints, wear painting clothes and just cover the floor in newspaper and paper and have fun squirting paint and doing hand/foot prints, or use different brushes, like ones for the walls or a toothbrush. My boy also loved playing with water, i would empty out a big plastic toy box and fill it with water, sometimes put bubbles or food colouring in and then give him things to pour with or fill like plastic cups, jug, pots with holes in etc. he would play for ages. he did get some really good toys that he still loves now (hes nearly 5 ) like marble run and a toy racing car you build and take apart with a drill. But like your boy he has has a wooden train set for years and never really played with it.