Have you tried removing half the toys he has access to? If you store them away and rotate them they'll keep his interest more readily than if he has them all out all the time.
Household items are usually a hit with this age - safe kitchen implements, saucepans and jugs, muffin tins, bathroom loafers etc
Your old handbags and any unwanted sunglasses, purses, ancient unused bunches of keys...
Empty cardboard boxes. Massive ones you can ask for at Currys, and turn them into houses or whatever, smaller ones they can put things in and out of
Dropping and throwing seems popular at this stage! You can make a beanbag with an old sock filled with dried beans (not kidney beans as they're toxic!) or balls, balloons etc
A living room disco with flashing Christmas lights and the curtains drawn is good fun on a rainy day
Stacking cups and blocks
Tents made with draped blankets
You can start to think about role play, treating teddies or dolls as though they were babies
Very simple peg jigsaw puzzles
Those pop up men, I think they're Galt?
Sensory play with sand, rice, seeds, tapioca beads etc
Musical instruments
One of those mats they can 'paint' on with water
You can make a sort of giant marble run with lengths of gutter pipe and a tennis ball
'Busy' boards (you can get quite dizzy with them if you look on pinterest!)