Well, don't put too much stock in the summer league. In actual games, he will have a well-defined roll, the floor will be more spaced, the paint will be policed realistically, teams will try harder to get back on defense, etc etc
He was a very good rebounder in college- he averaged about ten per game and in the big ten he was first in rebound rate his freshman year and second last year... to compare him to some big NBA names he rebounded less than DeMarcus Cousins but more than Greg Monroe did, if that helps.