This is a very interesting POV for him given that by the latest numbers I can dredge up he's currently 12.6% further ahead in the race due to the delegate system. He's received 30.7% more total votes than Cruz yet is ahead in delegates by 43.3%, this is entirely down to delegate allocation favoring the front runner in many state races. Source for the numbers here.
Knowing this, should the Republican Party heed his message and scrap the delegate system? If so, given that neither candidate will receive the required majority votes to clinch their party nomination, would the fair thing simply be to see Trump step into a runoff primary against Cruz alone? This is traditionally seen as the fairest way to settle non-majority election results in democratic systems.
Trump complains about "corrupt" delegate system