Depends. Is it a money map? Or a regular Blizzard type map?
If its a money map, just mass him via up'd scouts. They work well :
If its a Blizzard type map (not an endless supply of income) then it would be a lot more tricky. If there are just 2 islands, you and him, then most likely he used a lot of resources to make his fortitude. You keep try hit and run tactics.
Hit him, move back. Don't lose your units, but keep trying to take out his missle turrets. Eventually (which could be a long time) run out of minerals.
Or you could hit him hard with shuttles full of reavers by his CC and take his workers out. Then try the hit and run tactics.