Be frank with him and tell him it is up to him to find out what he wants to do now he has retired.
You are his wife, not his Nanny, so either he goes out and makes some new friends, at a bowling club, model train club or something of the sort, or findes some voluntary work that appeals to him.
He does not look like a dying duck in a thunderstorm because you are going out with your friends.
The day may come, I hope it never does, where you cannot leave him, as he needs twenty-four hour nursing. and he may actually hasten that day if he does not DO something.
All right he gardens, but he can't do so in the winter. You say he is creative, so surely it can't be all that hard for him to find a hobby.