I'm sorry to hear that your son is struggling with numbers. Maybe it's a matter of making numbers fun for him. For some kids, math and number recognition doesn't come easy. Have you tried playing games, making up stories about numbers, or finding fun ways to write numbers (with sidewalk chalk, etc.)? You could even try this joke out:
"Why is 6 afraid of 7?"
"Because 7 ate (8) 9!"
As a former 1st grader I have seen this a lot. If you haven't already contact his teacher. Otherwise, one thing I would do is to have your son use Popsicle sticks or other pieces of material and have your son make his numbers. He can "build" his numbers and they become his. Place these around the house and keep looking at them and labeling them. The best I know of is to constantly talk with your son about these numbers. While at the store show him the price, forget the idea of money and dollars and cents, and say "Hey I see a 3 and 5 and a 0. Wow that's pretty cool. Humm these numbers are everywhere." Keep showing him these numbers everywhere and do not be afraid to give him the answer. Just bring these numbers up in conversation and keep showing him the numbers in places other than just his math worksheet. Maybe make a song out of these numbers. Songs are great!
Just keep up the great work and try not to worry too much, it is common. If this continues to pose a problem by the end of 1st grade and well into 2nd grade, then I would meet with the teacher and maybe your doctor.
Good suggestions below. My wife teaches 1st grade. You need to make numbers concrete. Start counting objects from 1 to 5. Lots of practice is the key. Count anything. Cars, blocks, toys, clothes, glasses. Then go from 6 to 10. Practice practice.
Do not worry, he will improve.
One thing I've seen really help my son with numbers was that I got a box with lots of compartments at the hardware store and I wrote a different number on each of the compartments. Then I put that number of things inside (beans, buttons, pennies) and had my son count them. Sometimes it helps to have something physical in their hands to look at, while they're looking at the written version of the number (like the one you write on the box.)
It's an article on this site about learning disabilities with numbers. While it's not to say that your son has a learning disability, it's important to have some background and be sure he is being evaluated at school.