There are millions of craft activities you can do, and they don't have to cost much money. Start saving anything like old catalouges and magazines with colourful pictures, pretty shiny paper, sweet wrappers, leaves, twigs, little pebbles, shells etc.. He can make collages and pictures, decorate birthday cards, and have lots of fun just cutting things out (well,actually, most kids will happily spend hours just snipping bits of paper)
You could also try painting, playdough, potato printing, the list is endless. You could also do some cooking together. Try the library for some books on craft activities for children, they will give you loads of ideas.
Hope this helps.