Let your husband go with your blessing, it’s been his dream and he’s obviously fit enough and confident enough to go
Make sure he has a very, very good insurance because at his age if anything happened to him even a small trip, could end up in needing medical treatment which would cost the earth in USA
As for you, hopefully it’s because it’s a new diagnosis making you so scared, because think of the many, many people with health problems more severe than yours who are living alone and managing perfectly.
You will manage well if you allow yourself to. Accept any help offered, let neighbours ( if you’ve got good ones) keep an eye on you, if you have children, grandchildren, close enough to pop in, if not make an agreement to phone you each day.
You are probably a bit in shock from your diagnosis but you are not defined by your illness, you can live perfectly well for three weeks on your own, it’s not a lifetime and would be selfish of you to stop him going.
One last thing when he goes, if he rings you, do not go on about how scared you are, how upset, how ill, do not spoil his dream. pull your big girl Knicks up and tell him you’re doing fine.