How to play 101.
DUPLO
do you have enough of it? You need just loads of blocks etc. Go on ebay and get bags of 2nd hand.
Sit on floor and suggest things "shall we build a farm!" , " lets build a house for your cuddly bunny". Show him bits, get him started then follow his lead, let him develop it off into something else & roll with it. Its ok if he says its a dinosaur park and he then smashes it up because "it was T-rex".
BRIO
Tip out all the brio and build the longest track you can round the downstairs of the house.
Tell a story inspired by something like thomas the tank engine then act it out. "Uh oh, edward's hit thomas he's derailed! Quick james needs to come from the engine sheds over the level crossing and bring the crane"
BOARD games
Play orchard toys games with him
Choose games with two dice where you need to add small numbers and talk about it, get him to move your piece for you & count the spaces
IMAGINATIVE
base it on what will interest him. It may not be baby dolls. My son liked:
Sweet shop: buy cheap sweets in supermarket and put in old empty herb jars. Write a price list on a white board eg cola bottle 1p, snake 2p, lolly 3p. Give him 20p to spend and make him work the money out. Swap and he can be shopkeeper while you buy.
Cafe: write a menu, set up a kiddy table/box with a table cloth (i use an old cotton scarf!). Play at being customer and waiter and take turns. Use real food, make a big fuss of pretending to be a snooty french waiter etc. Do washing up in a bowl on the kitchen floor afterwards.
Draw pictures together. You can buy books that give you step by step "how to draw" for different characters which can appeal.
Other brilliant toys
BUILD IT - if he liked a tool set he will love.
Magnatiles - build a fort and defend against attacks from mummy with a straw and dried peas or those tiny pea size pompom balls
Boys can be quite different to girls at this age, my experience is that they sometimes play less independently and need you to engage/start them off/play with.