Here's the FBI's write up on Miranda rights
http://www.fbi.gov/stats-services/publications/law-enforcement-bulletin/february2011/legal_digest
CONCLUSION
The "public safety" exception to Miranda is a powerful tool with a modern application for law enforcement. When police officers are confronted by a concern for public safety, Miranda warnings need not be provided prior to asking questions directed at neutralizing an imminent threat, and voluntary statements made in response to such narrowly tailored questions can be admitted at trial. Once the questions turn from those designed to resolve the concern for safety to questions designed solely to elicit incriminating statements, the questioning falls outside the scope of the exception and within the traditional rules of Miranda.
In general, I don't see a big problem with that.
Regardless, like any accused, he has the option of saying nothing.
Reportedly, they already confessed to the bombing of the marathon to the guy they carjacked.
In this particular case, I don't think they'll need any testimony from him to get a substantial conviction so it's not likely to be much of an issue should it go to trial.