As you are travelling to BOS certain US laws over 'access rights' might mean you are able to take your rollator onboard I believe. Ultimately it depends on the size of it and the ability to safely stow it onboard, but having travelled with a disabled family member BA do have onboard wheelchairs on their longhaul aircraft. It could well be advisable to phone or email BA and ask.
http://airconsumer.dot.gov/rules/382short.pdf
Book assistance for both ends of both legs. BA do frequently get it wrong and just like EU261, deny everything until proven guilty. That's customer service for you.
http://www.reducedmobility.eu/201501...an-rights.html