I wonder what his muscle tone and core strength are like as well? Handwriting? Is he / was he a messy eater? Other gross motor skills like walking crawling?
I typically find children who drool do have muscle tone variation unless there is a different medical reason for this or it’s due to medication?
In regards to his speech at sequenced movement… what happens to his jaw lips and tongue when he starts speaking more quickly? I assume he isn’t able to maintain correct placement and sustain the movements over longer periods of time? Could this also be related to muscle tone or does he struggle to achieve placements when accessing multiple speech subsystems at the same time?
Once he achieves more accurate timing and transitioning for motor speech movements then his prosody should correct.
It sounds like he still has errors and PROMPT therapy weekly would really help him. Possibly in only as few as 6-8 sessions of done correctly.