You can paint with water! If you have empty paper towel or toilet paper rolls, those are great to paint.
Build a fort with couch cushions/blankets, if you have crayons or play doh let him play with those, make a sensory bin with rice, water, jello, crushed cheerios there’s tons of options, if it’s warm outside/you have a yard let him play in the water or find sticks/rocks to play with, chalk outside, editing to add you can make cupcakes or a cake together too and have him help decorate it or even cake pops
Make edible clay (made with flour and water)
Build a Bear (if you get birthday bear it only costs as much as the age your child is turning)