I'm thinking of replacing my husband's coffee with decaf without his knowledge for over a year
Do you think that trick will work?
I think it will, he drinks his coffee quickly.
Replacing your husband's drink without his knowledge is wrong. If you want him to drink decaf, you should ask him to and not trick him.
So you think it's wrong to try and help with his health?
No, it's fine to help someone. The difference here is that you are doing it without his knowledge. Tricking him is not going to help.