1) Let it go.. go with the flow. If he bumps you, take it as an opportunity to practice Ukemi and subsequently tell him how great his atemi errr I mean bump was and laugh it out. Aikido practice is just that practice... don't let ego come into play, your or his.
or just simply....
2) Challenge him to a GONG-SAU aka RAN-DORI, JIYU-WAZA, KUMITE or simply DOJO-BRAWL. Last man standing wins. Simple.
Agreed with 1)
Just go with the flow. If he really wants to "bump", strap on a pair of gloves and see him in the ring. Otherwise just keep practicing.