The Montessori way is to teach 1 to 1 correspondence, then quantity, then numeral, than pair quantity and numeral.

1:1 Correspondence: This means that he can count a group of objects without missing any or counting each more than 1 time. You can encourage this skill by having him help set the table: 1 fork for 1 plate, or 1 teacup to 1 saucer, anything that has him putting 1 thing w/ 1 other thing.

Quantity: Practice counting objects (beans, apples, pine cones, whatever) up to 5 and then up to 10. Make sure he touches each object as he counts it.

Numeral: This is what you are doing now. If he is interested, stay with this, but also try to encourage him to practice 1:1 Correspondence and counting quantities. He can write the number in the sand, trace a number with his finger on the carpet, anything that has him using his fingers to trace onto something tactile (we use the index and middle fingers to encourage the 3-finger grasp used for writing) . You can also trace the numbers on his back or palm with your finger or have him do "air writing": tracing numbers up in the air. After he has worked with these, move to the chalkboard to practice writing. If you have a large, mounted chalkboard or a large chalkboard on an easel, these work best.

Pairing numeral and quantity: After he can successfully count objects to 10 and knows his numbers 0 -9, begin pairing quantity and numeral. For example, make a set of numbered cards, 0-9. Then give him just enough beans to pair a quantity with each numeral: 1 bean to the number 1, 2 beans to the number 2, etc.

This should keep him busy for a year or so.