Use real objects for r-controlled keywords.
Add a tactile component to spelling by incorporating actual objects that represent vowel sound keywords.
Hopefully when you introduce a new sound, you provide your student with a keyword picture to associate a visual cue with the sound. You can take this one step further and also incorporate a tactile component by bringing in actual objects to represent the keyword.
When practicing spelling words, have the student choose and pick up the object that represents the vowel sound they hear in the spelling word BEFORE they start spelling it.
Have them then give the vowel spelling based on the object they selected (ie, if they picked up the car, they should state that a-r spells /ar/ like in car), and then spell the word.