Whenever you see him about to go, start associating a word. You can usually tell when they are about to go... Most will step backwards and shake their tail, but yours may do something different. Watch out for these signs and say the word as soon as it happens.
However as Michael has said, he is probably too young to completely learn it but it never hurts to start early. So yes, he will make a mess so live with it for the time being

Once he is older (people usually recommend 6 months to a year), then you can start designating specific places for them to go. When you see them about to go, take them to that place, say your word and when they go, praise and reward as you do for training. They should eventually pick up that it is the spot for them to go...
Good luck!