He can hire an agent and come back as long as he requested a draft evaluation.
But since he was not invited to the combine he must decide to come back before the draft. Only players who are invited to the combine can come back after the draft.
Where did you see this? I'm not saying you are wrong at all (actually hope you aren't), but I haven't seen this anywhere. It does make sense.
Here is a summary from Bleacher Report and this is similar to everything I've read thus far:
"Eligibility after the NBA draft
This is another common-sense rule change that the NCAA has made needlessly complex. The easiest solution here would be to say that college players with remaining eligibility can return to school within two weeks of going undrafted. Instead, the NCAA has narrowed the potential pool of players to those who participated in the NBA Draft Combine and given them only four days to decide.
This is a protection for players who have either been fed bad information or made a misinformed decision about their NBA standing. For them, being able to return to college and pursue a degree is a major boon. However, because of the combine condition, only a percentage of the players will benefit."