The record is currently held by Claude Gregory at 904 from 1978-81. Happ had 278 in his freshman year, which was also the 9th best single season tally in Badger history (record is Jim Clinton 344 in 1951).
To get to 905 Happ would have to average 905-278=627/2 seasons=313.5/35games (assuming same number of games as his freshman year)=9.0 rebounds per game.
Happ averaged 7.9 rebounds per game as a freshman, so an increase of 1 per game doesn't seem unreasonable with a small increase in minutes played in his next 2 seasons over his freshman season.
If Happ were to average 9/game for the next 3 seasons(35 games played average), he would end up with 1219 career boards.