Ok one way to boost his disposition by an extra 10 points is to talk to him with your weapon drawn, persuede him up to max, exit dialogue sheath your weapon then talk again.
His disposition should be 10 points higher than before.
As for charm, have you tried ranged and touch versions as sometimes I've had trouble with the spell taking effect, and don't forget use several versions of charm spells.
Don't just cast the same one over as it won't stack if you just recast the same spell multipule times.