Upvote:2
Here is an F-1 travel FAQ from Homeland Security which mentions:
If USCIS has approved your OPT you will be expected to have your EAD in hand to re-enter the United States, in addition to your Form I-20, valid passport and visa, and a letter of employment if you have one. If you exceed the limits on unemployment while outside the United States, you will not be eligible to re-enter the United States in F-1 status.
If he has all of these things including proof of a job in the US, he should be able to re-enter for the purpose of finishing his job before his OPT ends. If he doesn't have a job in the US, he might have a harder time explaining his purpose of entry.
Before traveling during OPT, it's always a good idea to check with your Designated School Official to make sure your I-20, SEVIS record, OPT reporting and STEM OPT reporting requirements are all in order.