Our small family of three needs to watch our diet. We have casein allergy and gluten and soy intolerance. The casein/no cow dairy we try very, very hard to abide by; since we don't have Celiac disease, we allow the gluten to slide once in a while, and the soy we try to stay away from (which is almost virtually impossible, since it's used as a binder in almost everything).
Yes, I'll admit, we cheat. Very seldom, but it happens. Since we don't have immediate outbreaks or need to go to the emergency room, those around us don't know when the food we've eaten affects us (since it's usually a day or two or four later). Eating dairy will give my husband diarrhea and me constipation. Eating gluten and soy affects my skin as well as my immune system (I had severe malabsorption and Leaky Gut, which I'm still working on repairing; it's been 4 years). My daughter has Sensory Processing Disorder; keeping her away from these foods helps her function better. She's high functioning, though, and except for some social issues, one would be hardpressed to know she has "issues."
Getting the extended family to understand our needs and abide by them, though, has been like pulling hair out.
My MIL (and actually, my husband's side) has been the best about this. She really tries. My daughter spends a week with her in the summer (we live 5 hrs away from her). We send GF bread, goat butter, and alternative milk as well as GF treats along with her when she goes to visit; she does eat a little more gluten than she normally does when she's with Grammy, but overall, Grammy tries her best. She's in her 60s, and having to watch food and what's in them isn't something she's needed to do in her life. But she currently suffers from Pancreatitis and needs to watch her own diet, so she's aware of the food/bad health connection.
My folks, on the other hand, don't look at or read the ingredients on anything, they just eat it. My dad says he's going to die soon anyway and there's no reason for him to start eating better now (see what I have to put up with on my side!). Whenever we go home to my folks, we bring along some of our own food. However, since we don't live there (6 hours away), we end up eating whatever they're eating. That means, they make it, we eat it, and there are no concessions made. At Christmas time, everyone brings food to eat/pot luck; again, none of my siblings do anything to change or augment what they make and bring to take into account our food issues. And we've mentioned/noted what those are numerous times.
We have learned to suck it up, eat the food, suffer for a few days, and just let it go. Since we only get home to either side maybe 2x/year, it's not worth fighting over because they just. don't. get. it. And they don't want to learn about nutrition or what's really in their food. We've tried to educate them, but it literally goes in one ear and out the other with nothing registering in between.
Since our food issues don't affect us long term (well, other than with immune issues), we let it go because it's harder to fight it. I console myself with the fact that our parents won't be around forever and soon, we won't be going back home at all. We take loads of supplements and make sure we have supplements to counteract what we eat when we visit either family, say a prayer to our Spirit Guides to protect us and our body elemental to negate anything bad in what we eat, and let it go.
In your case, though, with a son who has autism who is clearly helped by following a very strict nutritional diet plan, I feel it is completely and totally disrespectful of family members to not follow your son's special diet plan completely. I don't know how badly what he eats affects him, but a week seems like that is about right for detoxing.
It all depends on whether you want your son to spend independent time alone with grandparents/family members and how much you're willing to put up with his behavior from eating "forbidden foods." And if your family members are willing or capable of change--which doesn't seem likely. My daughter knows when she's eaten enough "bad food" that she'll specifically request no this or that; I'm not sure if your son is capable of regulating his own food/eating, but hopefully, that could be something he could work on and voice, to advocate for himself.