The answer is ...who knows? Can he win Boston? Maybe. In terms of winning times at Boston, haven't we learned anything about the marathon? Especially marathons like Boston and NYC? anything can happen!
Some "lousy" Boston winning times during the last 12 years:
2005 Hailu Negussie 2:11:44
2007 Robert Kipkoech Cheruiyot 2:14:13
2012 Wesley Korir 2:12:40
2016 Lemi Berhanu Hayle 2:12:45
Not to mention the plethora of 2:10's and 2:09's. If you take a look at the data, since 2000 winning below 2:09 was the exception not the norm (only 6 times in 17 years).
If the conditions are Rupp ripe, he has a fair chance to win as any of the other elite runners. (IMHO)