Desperate times call for desperate measures. There is a reason why he asked you and not some random female. Perhaps you could ask him.
Are you guys jerk off buds? Have you played sports then hit the showers and are comfortable naked with each other? Seen each other naked during sleep overs or trips without any awkwardness? Also, as both you guys are "straight" are the any XGFs in the picture that would be too awkward? Does he have trouble with the ladies? Is he insecure about his dick? Have you already helped him pee and/or get dressed/undressed when you visit? I am not asking for and do not need the answers. But if any of the answers are "yes" that just may be some of the catalyst for the request and reasons to consider saying "OK".
Certainly some dialogue would help and if you should proceed it should be understood he owes you BIG TIME. And if there are repeats before complete healing, that is additional paybacks, none of which need be sexual. I agree privacy is key (duh?) if you have your own place and no roomies that would be best, otherwise timing it while mom/family/roomies are out for a few hours is a must.
If it is too much for your boundaries then do not do it and explain it to him. That also could harm the friendship, if he feels he'd do it for you and, through his obvious frustration, begins to resent your refusal. It does not have to be as weird as some here suggest. If you feel up to it then, it happens, you guys move on, and when you are older you get to laugh about the time he broke both hands... Just another chapter of good friendship and male bonding. IMHO
And now my joking side: You tell him every time you jerk him off he's got to blow you right afterwards! LOL Sorry, could help myself:redface:
Good Luck