M.P.
Without knowing your mil and based only on what you've written I too would hesitate sending him. After I read Laurie A.'s response I have a different thought. I spent a month every summer with my grandparents from the time I could first remember (probably around age 6 or7) and loved it. We lived in the same town as them until I was 5 and so I knew them very well.
My mother and grandmother didn't much like each other back then and I think that's the reason we moved. But......I wasn't aware of their feelings until I was much older. My grandparents were clear with me that they were grandparents only. My parents and my grandparents were in agreement about my care.
If you trust them to keep him safe and have a firm agreement about the meds, and he doesn't miss school, I would let him go. You can keep touch by phone quite easily.
It's too bad you don't like the husband. If he's retired military, he probably has a strong code of ethics that I would admire. Your son may actually benefit from a more controlled environment with definite bed times and expectations for clean hands, clean his plate, etc. Often children who are anxious thrive in such conditions. This is not a comment on your way of living. It's a comment, that while you may resent such control, your son may not.
I don't know what you mean when you say he's militaristic. If you mean he's rigid and lacks patience that is a different matter. You have to trust your judgment. Since he's often not around, do you know how he would treat your son? :You said you trust him which sounds like he's OK with your son.
Your MIL sounds like what my crowd calls an "air head," meaning that common sense just isn't there. I'm guessing she needs her husband to keep her grounded and I'd feel more comfortable having him with them.
I'm guessing that if her husband knows your son is to have meds, he'll see that he gets them.
Anyway, I'd seriously consider letting him go.
Re: staying over on a school night. My grandkids don't stay over on a school night, as a general rule. At first I felt hurt, feeling like my daughter didn't trust me to get them to bed on time and to school. With experience I realized how much work it takes to manage them on a school night and am glad she set that rule.
My grandson is now 7 and did spend Wed. nights with me during the summer. He still spends Wed. nights with me and it's working out OK because we've established a routine. Being here isn't disruptive to his school routine. But I would not want him to stay over on a school night now and then. Perhaps, even tho she was upset at the time, she's more understanding now. Having him for 3 weeks will help her understand it, if anything will.
School is important and I would not like that she's not willing to consider it important. Perhaps she doesn't realize how much more information there is to learn in comparison to when she was in school. School attendance is much more important now than when I was in grade school. Missing a week would be stressful on a little boy already feeling stress.
And I absolutely would not like it if anyone talked with my child about doing something before talking with me. It was natural for me to talk with my daughter before making plans with my grandchildren. She goes one step further and requires that I tell her what we're doing while they're staying with me if it's something out of the ordinary and even if it's a spur of the moment plan to do something that will take a couple of hours. I have to respect her request tho I sometimes forget to call. The last time was when I took my 10 yo granddaughter ice skating.
Your mil has put you on the spot with your son. I'd have my husband talk with her about that and tell her to not ever do that again. Actually, both of you talk with her at the same time.
You are not being selfish or over protective. You're being a sensible mother who is aware that this decisions needs careful consideration.