Try to make learning numbers fun. I personally feel for most 5 years olds that writing numbers five times each is drill work. (Especially if you are making him write many numbers during the same session. ) You could have a number strip or number line up in the kitchen and every time he is excused from the table he needs to point to a number and say what it is. He could further tell you what number comes before it as well as what number comes after it. (The numbers are a daily visual that he will see when ever in the kitchen.) You could play hide and seek with numbers and when he puts them in order he could earn a small reward. You could practice the 1-10,11-20, 21-30, etc... so it's not overwhelming. Incorporate numbers in your daily talk.
I love Lakeshore learning in Palatine for educational products. There are other good learning/teacher stores that I have used too. (I am a former grade school teacher who is now at home with my 5 year old twins.) Librarians are also very resourceful. Ask his teacher, too, because she knows his learning style and can offer you some advice on how to help him learn his numbers. I feel the hands on approach to learning is best. So make sure to include activities that have him actually manipulate objects to understand math concepts.
Good luck! J.