Sorry to hear your husband has been ill -- Lyme disease can be very challenging!
First, I'm sure you have already run this past his physician. But if not, please do. You indicate he hasn't had time to adjust to his meds - the doctor may want him closer until all is set.
For things like fatigue and pain - WDW will recommend he use a wheelchair or
ECV. This will allow him to have a seat whenever/wherever he needs one, and will help him to conserve his energy. He does not need to remain seated at all times; the ECV or wheelchair can be parked if he wishes to walk and stretch his legs a while. The majority of attraction queues are accessible to
ECVs or wheelchairs, for those that aren't you'll be directed accordingly (a few will require him to transfer from an ECV to a wheelchair provided at the attraction).
Vision and hearing concerns will need to be dealt with at each attraction. Just mention his needs to each CM you encounter along the way through the queue.
I suggest you plan for avoiding the sun with a large wide-brimmed hat and protective clothing. Most of the queues and attractions are indoors and air-conditioned, whereas most of his sun exposure will occur outside between attractions and moving around the park.
You certainly may speak with Guest Relations about a
DAS. If qualified, this will allow your party to wait somewhere else, but it will not reduce the wait time and he will need to find somewhere out of the sun (while the lines may be shaded). Only Guest Relations can decide whether anyone qualifies for DAS, but he will have to explain his needs and not his diagnosis.
Enjoy your vacation!
