Stacey,
Unfortunately, spelling is a natural ability pretty deeply tied to visualizing words. The good news is that studied dictation is very focused upon seeing a word spelled correctly and doing the work to get the correct image of a word in and the incorrect image out.
I would be inclined to suggest that you move toward doing studied dictation daily with your son who is struggling with spelling. You could do it this summer as one of your only subjects. You will see improvement, but steady practice daily will make that happen. Steady practice even in high school reaps big rewards. You could do a different spelling program, but being a recipient of that in your school days you can see that spelling programs aren't all they are cracked up to be!
I would lean toward steady practice in dictation and helping him proofread his written work as a means of focusing on the issues. If you don't have time to proofread his written work, you could have your strong speller help proofread. Underline in pencil anything that is not spelled correctly. Then, write the correct spelling in pencil in the margin. Have the child who is working on spelling, erase the incorrect word. Then, have him copy the correctly spelled word from the margin. Last, erase the word in the margin, leaving only the correctly spelled word in its proper place in the written work. I know there may be many words in the margin at first. Over time, this strategy combined with studied dictation does help. It just takes a long time to see fruit!!
Blessings,
Carrie