You have to find out in which areas he is stronger, in which weaker.
Is he behind in military?
Either form an alliance with another civilization and capture some of his cities or expand aggressively and get more cities to get an advantage later.
Is he behind in tech?
That means he has slower research, focus on increasing your advantage so you get units that can kill 3 of his units on their own.
Is he behind in economy
Increase your advantage with trade routes and make some good trades with other civilizations. Use your money to buy core buildings/units to speed up your development.
When all else fails...
... bribe the other civilizations to go to war against him without you and boost them with money and strategic resources. Make sure your minions... uhm, the other civilizations are not so weak that they get taken over by your opponent, making him twice as strong as he was before.
Go to war against the weakest or one of the weaker civilization, take over all the cities and come out twice as strong yourself.