There is probably no other way than by gently drilling him from time to time, checking and supervising his homework and reinforcing a spelling rule when you can. Ask his teacher to send home lists of words they are working on in class (no more than 10 at a time) to review with him.
I personally think that spelling, because of spell check on our computers, and simple math calculations, because of hand calculators, get short shrift in today's educational systems. You have to help out at home. If you do, you won't have to worry about him falling behind.
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