There are different ways to look at this. It can indeed be demoralizing to see coworkers slack of all the time, when others are busting their asses off. Howev

There are different ways to look at this. It can indeed be demoralizing to see coworkers slack of all the time, when others are busting their asses off. However – before mentioning this to a manager, you should contemplate if anything good can come out of it. There is a non-zero possibility that nothing good would come out of it. If it is the case that he’s slacking off, his manager should notice that after a few weeks/month. And if the manager doesn’t notice – what does that say about the company…? If I were you I would focus on the bigger picture. Are there other, good coworkers? Are your tasks fulfilling? If the bigger picture is fine, maybe you should just turn a blind eye to this. And if the bigger picture is not fine, maybe it’s time for you to go on to new adventures.