That's a huge question! And simply put, it would take a long list of books depending on what he is having trouble with and what his learning style(s) are.
Do go to the teacher. And like the other responses have said, ask her how she is teaching concepts.
Do ask what the titles and publishing companies are for each subject. Today there are online components for each text and there is usually a child/parent section that may help
Do go to a teacher's store. Ask for recommendations that would teach the same concepts as what your child is learning.
Do break every task into its smallest parts. If he is working on a math worksheet get tangible items to count like pennies or crayons, pasta, etc... Solve each problem using these items so he can 'see' the problem.
If he is working with reading. Do a picture walk first. That means, look at the pictures the first time through while ignoring the words. Talk about the story, plant the vocabulary in his head while you talk about the pictures. Then after you look at the whole book, go back and have him read it. Only ten percent of the book should be work. If he brings something home that he is stumbling on every word, you read the book to him. It is better that you read it to him then having him get frustrated.
Do make homework fun. Break it up into small ten minute sections.
Do make sure he gets a snack before.
Don't let the situation get stressful. If you find yourself getting frustrated, stop and let him play for awhile.
Do pat yourself on the back. The fact that you want to help your son and that you are trying to find an appropriate way is huge!