Would you recommend working at sound level at all? What about oral strengthening exercises for the dysarthric features?
If the child has a large enough repertoire of sounds there is no need to work at the sound level. Take him to the next challenging level. If I am assuming correctly, that “oral strengthening” exercises are the same as oral motor therapy, then there is no evidence that “oral strenthening” exercises are of any benefit. Rather, I would observe and note which sound transitions are challenging for him and then work on those transitions. For instance, let’s say that he has difficulty transitioning from /k/ to /t/ as in “back to” I would start out asking him to repeat /k/ — /t/ one right after the other. If this is easy, move him on to two word combos: take to, make time, etc. Short sentences would be the next step. The idea is to determine the level of breakdown and work from there.