1) The others will not ask him any question - because each candidate will be allowed to ask only 1 question.
2) He should not ask anything other than economy (may be spending in connection to foreign policy) which is his strong point and the others' weak area. Anything else will be easily answered and will be specific to 1 candidate (like immigration - which is specific to mccain, i think romney will ask that anyway.)
Yep, my bet is that they ignore his presence entirely. Or they ask him the question about truthers or white supremecists.