It doesn't really sound like mom is trying to pull a fast one because so far she has been quick to come pick him up but I do think she is not allowing him to FULLY rest and get over whatever he has and that's why he isn't all out sick but is still showing little signs of illness.

I would require him to stay home until he has been 100% for a FULL 24 hours or he will never kick this thing. Plus, he will continue to expose the other kids and you and your family too so I would err on the side of caution and tell mom that he can't return until he is 100% for a 24 hour period of time.

Seems it would be easier on mom to take a full day off rather than to continue cutting her days short everyday this week. Know what I mean?