It sounds like a PROMPT approach would really help and an analysis of the motor speech sub-systems during informal and formal speech samples would be useful.
I think you’re right, it sounds like he is working hard in his cognitive- linguistic system to utilise other strategies such as slowing down to support the difficulties in his physical sensory system. This also possibly impacts his ability to organise language??
From your description it sounds like he is compensating for his poor motor planning by slowing down and using other strategies. The motor speech movements aren’t efficient and you are right that this does impact on resource demand for other linguistic demands and fluency.
The poor motor speech planning can also impact prosody and rhythm.
How is he academically at school? Have u observed other difficulties in his cognitive linguistic domain that might suggest possible language difficulties unrelated to the physical-sensory?