Look up Jeremy Bloom. He was a professional downhill skier who then accepted a scholarship to play football at Colorado. He went into a legal battle with the NCAA and was declared ineligible after his second year. However, he was trying to continue to collect skiing endorsement money while playing football at Colorado.
Here's a document on amateur re-instatement:
Also, of note from this document: http://www.allbusiness.com/legal/contracts-agreements/4084260-1.html
24. Each NCAA division adopts its own operating bylaws, which provide rules and regulations consistent with the NCAA Constitution and relate to the administration of the division and its championships, the delegation of authority, and the procedures for enforcing the provisions of the Constitution.
25. The amateurism provision in the Division I bylaws prohibits pay for various types of athletic performance in excess of necessary expenses.
26. This same provision, allows a professional athlete in one sport to represent a member institution in a different sport.
27 Student-athletes are not, however, eligible to participate in intercollegiate athletics if they receive endorsements of any kind, even if for a non-NCAA sport.
Also, from http://sports.espn.go.com/ncaa/news/story?id=1796372:
Rush has filed an affidavit from San Diego Chargers wide receiver Tim Dwight in support of Bloom. Dwight said the NCAA allowed him to run track at Iowa and keep money from endorsements he made while playing for the Atlanta Falcons.
The NCAA said the cases are different because Dwight stopped accepting endorsement money when an NCAA reinstatement process began.
So it looks like as long as he doesn't accept any endorsement deals and competes in a sport other than his professional sport, he should be okay. However, I'd consult with a lawyer and/or NCAA eligibility representative over my interpretation.