He can do plantar flexion and dorsiflexion of his recovered foot. It is like pressing the foot downward and extending it upward keeping your leg straight during sitting position. He can move the foot side wards also keeping his leg straight. You have to start this only after full recovery and getting a go ahead from his orthopedician.
He has to do this every couple of hours till his stiffness decreases and if he has pain and tenderness then consider some ibuprofen for few days and also apply ice packs to give him relief.
One good exercise that I know about is to have your husband write the alphabet with his foot. If can have him do it in a bucket of warm water as this will help with stiffness.This is needs to slow and controlled about 1 sec per letter. (When he writes make sure his foot is up at close to 90 degrees as possible this will keep the Achilles from being stressed. I know from experience of my doctor pointing out that I was doing it wrong. Once I started doing it right the Achilles did not hurt as much) He can start out small and as his range of motion improves he can make the letters a little bigger. I would do this about three times a day and I do about 3-4 sets.
I hope this helps,