They also need to start realising that a number relates to a definite QUANTITY, where as a letter only has meaning as part of a word.
So if you make some games, like large floor tiles of numbers and he has to, (firstly) lay them out in the correct sequence, and (secondly) place the appropriate number of small objects on each one (Smarties, or Lego bricks, etc). If using bricks, they can be built into towers, showing more clearly the increasing quantities. Child could extend that, if he has pencil skills, by drawing a bar graph of the numbers.
Start with 1 to 9; then introduce 10, and if understands that, carry on with two digit numbers up to 20. Maybe use different colour bricks to show how the 'teen' numbers work.
A 'washing line' with clothes pegs and paper numbers is another way it can be done in school. If he has thick felt tip pens, or crayons, he can try drawing the numbers himself.
Also, eventually, introduce Ordinal as well as Cardinal numbers:
www.mathsisfun.com/numbers/cardinal-ordinal-chart.html