You could flash larger numbers.
Or create flashcards and play some games with them.
http://www.printable-flash-cards.com/flash-cards/free-printable-number-flash-cards/ Maybe a memory match game for a young one.
You can also print out a 100's chart and laminate it .
http://math.about.com/library/100.pdfMy 4yr old likes to take a black marker and make x's on the numbers. You could use pieces of cereal and candy too.
If your super creative and have lots of space you can make some nice sized arrows, write the numbers on them, and then laminate them ( to keep them nice). Hang them low enough your son can point and touch and make a big number arrow. Or some other shape if you wish.
The thing is being consistent, and practice every single day.
Those are some of the things that have worked in my house.