I am not sure that I am adding much help and may sound confusing, but fwiw, I'll comment on two things:
Spelling Power: We have this - I found a used copy for cheap and don't do all the activities. We do really just spend about 5 min a day on this and I don't have to plan a thing. We go over the rule (my son does write it) and then work our way down the list - just spelling, nothing else. I like that it is so straight forward and not a lot of bells and whistles and extra writing - we do enough writing with other things. One thing I like to do sometimes with spelling if it seems like the writing side is overwhelming is to use foam letters (I have a set of Lauri ones) to spell with instead of writing. Sorta makes it more fun
Speech: I have twin boys who have a whole boatload of speech issues. We did have our friend who is a speech therapist evaluate them and help me have a sort of game plan with them. I bought a lot of speech books, cds, etc. to use with them and here has been our experience - it was just one more thing to plan and we didn't do it. I still like to read through the books with them and listen to the cds, but what has really worked best, and what our friend/speech therapist has said, was to be PATIENT (not my strongest virtue) and work with one thing at a time, usually it is a sound they discover themselves that they want to work on. Last year at their annual checkup the ped. said we needed to address the speech issues (I agreed) - for the most part over the past year we have been very laid back, no pressure (because that just frustrates everyone), and haphazardly gone at it when we recognize it. We get really excited when we accomplish a new sound. We have all been less stressed, and this year at their appt, he didn't even mention speech - I asked him what he thought and he said he thought they had improved alot, and many of our friends have said the same. I think on this issue in particular for us, being very gentle, patient, waiting on their cues of readiness (even if they didn't seem to go along with the "appropriate age" for being able to do whatever skill), has been the best approach - same goes for reading with these two.
And I don't know if the same would transfer over to spelling with your son, but with my boys, not being able to articulate certain sounds doesn't seem to get in the way of them sounding out words and being able to spell some. They can't make the "R" sound to save their lives (it is WAY down on our list right now) but when they are sounding out words with "r" they just make their "r" sound and move on. They know what it is supposed to sound like by hearing everyone else pronounce it correctly, it is just that they can't pronounce it correctly. So, I don't know, it could be that the speech may not get in the way of being able to learn to spell correctly so long as he can hear it pronounced correctly. I have found too, that with my boys, working on reading and sounding out words has actually helped their speech in causing them to concentrate on individual sounds more.