If they speak in sentences, can you get them to record them (talking tin lids?) and then get them to write them out as they play back.
Model with cut up sentences – this could be done on IWB as a drag and drop activity (may help him engage).
Do sentences as Cloze then build up the number of words (s)he has to add from a list and then own choice.
Group / shared writing (I’m sure you do this anyway).
Find what he is REALLY passionate/obsessive about and write about that (even if it is nothing to do with what the others are writing about).
Good luck