I think he can use EAD to work part time
Since the time he starts to use EAD, he begins AOS which is associated to your petition. However, if he stays at home without H1B and without using EAD, he may be out of status. Theorectically he can use EAD to work in other fields in short terms to activate his AOS.That's my understanding.
I understand you want him keep H1B.