Just communicate with him and let him know that you may not be able to handle all of the things he has planned for you due to your lupus. Tell him not to be disappointed if you can't do it all and tell him that you really appreciate his thoughtfulness and the fact that he went to all this trouble for you. This way he has a forewarning and a chance to think about what you are telling him. Then go ahead and do what you can and don't feel guilty if you can't do it all. Remember it's not your fault that you can't be and do everything for everybody.
I have grappled with this problem for the last 5 years or so. Not only am I getting older and running out of steam, but I have lupus, fibromyalgia, the yeast connection, high blood pressure, many slips and falls in the past, and other health concerns. I think it's remarkable what we can do with this illness.
There are so many people here that are so brave, courageous, selfless, and optimistic on this website that I am in awe.