Depends on your role. Assuming you are their direct supervisor, simply give more tasks. Slacking off is usually due to not enough to do. Follow the tasks at a high level, make sure he knows he’s being tracked. This has even the effect of increasing motivation, as most people value attention.

If slack off means he’s not showing up at team meetings, constantly missing deadlines, low quality of work, etc.. just give feedback on a case-by-case basis, ask how he would make it better next time.

If he fails to improve and the examples pile up, time to make hard decisions. I have to say, in my experience as a direct supervisor, only 1 out of like 4 people fail to improve, if you are willing to invest your time and effort.