You have already violated the law and this applicant knows it better than you. The ADA is not what applies here, but the FHA guidelines. These are two sets of laws that apply to rental property.
The FHA state that you must accommindate a service animal. What you should have done was ask the applicant to provide information from the applicants health provider that the service animal is indeed a service animal. That's all you can inquire about. If so, then treat the applicant as you would any other applicant for credit and background check.
You should learn the laws as to service animals, they are not classified as pets. The applicant can file a discrimination claim against you for denieing the service animal. This could be a very costly mistake on your part. Not only is it expensive to defend but win or lose, you will have a lot of paper work to do for the next three years.
Pehaps, it's best to call this applicant back and follow through to see if the applicant does qualify to rent. Meanwhile learn the laws as to service animals. Mass also has its own Laws as to service animals.
Maybe this can be worked out before you wind up with a complaint filed against you.