Most children who fall behind develop a poor attitude about what is going on academically in the classroom. You'll see a positive change in your son's attitude after he masters the basics and is better able to keep up with his classmates next year.
In math, start with the basic addition facts, then move to teaching subtraction, and finally, multiplication. To give your child confidence, you might start with facts that he knows well, such as 2+2, 2+4, etc. We have written two books published by GoodYear Books: Helping Your Child with Mathematics (addition and subtraction) and Helping Your Children with Mathematics (multiplication) that will show you how to teach your child basic math. These books will let you have fun with your son through hands-on activities appropriate for his age.
As for spelling, choose 20 words each week from the teacher's list. Quiz your son on the vocabulary, then concentrate on teaching him just those words he doesn't know. Be sure to review frequently the words he's learned.
For tutoring to be successful, schedule a definite time to work with your child each day, and keep your tutoring sessions short. Remember also to keep reading to your child every day, since he enjoys and is benefiting from it.