If possible, blow out the sky while keeping the actor (and his shirt) properly exposed. Then use the Extract effect to key out the highlights. Of course you may need a pretty big setup to achieve this using silks and/or flags. Not sure what the budget is for your shoot and how feasible that would be be for you.
You could rotoscope the actor using masks or the rotobrush, but if the actor’s shirt is blending with the sky, that would be incredibly tedious.