One of my DD's does poor on spelling tests. Her spelling grade is about half based on homework, though, so it averages OK. She is a kid who is good at almost everything, so we've worked as much on accepting that spellling is difficult for her at this time as on learning to spelling.
She has 25 words per week which often have a theme but don't follow the same pattern. It's a stupid list and she finds it overwhelming. She has a lot of spelling homework, but it doesn't teach her how to spelling the words (may be it works for the other kids?).
We focus on the first 10 on Monday, the second 10 on Tuesday, the last five plus 5 I chose on Wednesday, and Thursday is the only day we tackle the whole thing. Breaking it into sections has helped her stay focused and positive while we work, and oddly she is doing better overall, even though we no longer study every word every day.
When we study a word, we start by just reading it and thinking about it. We underline letters that are working together (such as "th" or "ar"), notice double letters, etc. Then I cover up the word and she spells it outloud. If she has trouble spellling it outloud, we go back to step one. Once she can spell all 10 words outloud, we get out a piece of paper and practice with her writting them. This is an important step for her, because for some reason even if she can spell it outloud, it may look funny to her when she puts it on paper.
Good luck and stay positive! And remind him that how he does on a spelling test is not an indication of his value as a person, how smart he is, or anything else that actually matters!