I don’t think you need to go back to ‘phonics’. You could teach/review sounds of vowels/digraphs as he does his spelling. I have a spelling book called “Natural Speller” which uses sight and sound patterns for the list of words being studied. It also includes some grammar skills. If you use sight and sound patterns in your spelling lists, your son can see/hear what vowels/digraphs and sounds go together. I would say doing that is basically teaching phonics.
My daughter has a learning disability and there is no consistency to spelling. She can spell something correctly one day and then spell it incorrectly the next. We are currently using Spelling Workout. It has more activities for practicing the words. They also use sight and sound patterns in their spelling lists. When I showed Spelling Workout to my evaluator, she really liked it and thought it was a good program for my daughter.
If you are interested in Natural Speller, I am selling mine. I bought it new and hardly used it. It’s a good spelling program, but given my daughter’s learning difficulties, I will not be using it. My email is psreitmom4 at gmail dot com, if you are interested.