I wouldn't run to your boss and say "Jim isn't doing his work".
Instead, before you both start working on the task, send out an email dividing the work. Do your share of the work and wait. When the deadline gets closer (not too close to the deadline, but close enough that it should be done), tell your boss that the deadline may not be met because you are waiting on some work to be completed.
If he asks which work is not complete, bring up the email where you divided the work, and show your boss exactly which parts are not completed. The best thing you can do in these situations is not to make it about the person, but make it about the work.
Be factual. Ensure you have left a reasonable amount of time for the work to be complete. People all work at different speeds. As long as you left enough time, and you aren't outright blaming someone, you should be fine. Once you start pointing out specific people aren't working, you kind of look petty. If you just point out that X is not complete, then it's completely about work and factual.
From your perspective, you shouldn't care (or in some cases know) why the work is not complete, and it's not your job to care why. It is however your concern that the task be completed on time, hence why you should only mention the work and not the person.