My son had almost the opposite problem.
His muscles were so tight that he could hardly move. I think he also had a buildup of lactic acid in his muscles because it was very difficult for him to walk very far.
One thing I've learned is that, sometimes, opposite reactions are due to the same cause. Like too much of a vitamin can look the same as too little.
Celiacs can be people with either constipation or diarrhea. The same diet fixes both things. Figuring out the best diet for your child is the best place to start - in my Mom to Mom opinion.
Food sensitivities were the answer for my son. We began with a total elimination diet just before he turned 3.
NOTE: removing one food would not have helped him enough for us to have seen a difference. He would have still seemed 'sick'.
We had to remove ALL grain, nightshades and lilies from his diet as well as refined sugar. Eventually we removed dairy as well and noticed further improvement.
After about a year we started to add some of the troublesome foods back in.
The only things he cannot tolerate at all is gluten. He still limits other grains and sugar. Dairy is brought in for birthdays and Christmas in the form of cheese for pizza but the rest of the year we don't consume it in our house. (Other than the odd chocolate bar that is 'won'.)
Doing all this with him made me realize that *I* do better on this way of eating too and am moving better now (43) than I did in my 20s. My joint issues (knees, lower back) are definitely related to dairy consumption.
The only milk we drink is almond milk. No animal milk.
We don't consume whole grains either. Whole grains tend to knot up our shoulders.
We've been doing this for 7 years now. Happily, his range of motion is better than ever. He does martial arts and slowly, very slowly, his kicks are getting higher and higher. And now he can run like the wind and it's wonderful to watch him run because he always has a HUGE smile on his face.
Last fall, we took to doing multiplication drills and then after each drill, I would chase him around a specific running course (about half a city block long). Five times tables meant five running drills. He outruns me, usually, for the last two.
- To think that he couldn't even walk 1/2 a block and now outruns me on it, is thrilling.
Our deal is that if he escapes me for 3 out of five runs, he wins a choc. bar. (gf of course - in Canada, it's usually and Oh Henry or a Reese peanut butter cup - I'm currently working on making a homemade, dairy free choc. bar though because I'd like to get that last bit of dairy out of our regular, every day kind of lifestyle.)
So, definitely worth trying but make sure you do it 'right' or 'fully' - or you might miss the boat that you were supposed to get on. Removing one food at a time... if the child is sensitive to 10 foods, at the end of a year you'll be so sick of 'altering' your food that you will eventually throw your hands up and say 'forget it'. I've seen it happen many times. Plus, even if you do have the wherewithall to stick with it, you're prolonging your eventual goal to healing.
I suggest reading "Breaking The Vicious Cycle" by Elaine Gottschall. It's a great book and the theory behind that diet has done some amazing things for challenged kids all around the world. Our initial diet was an altered version of that one. pecanbread.com is the website if you're interested. I have no affiliation with them. I just like their message and the results.
Some of the stuff you say about your child makes me think B12 deficiency, magnesium deficiency, etc. I'm not saying to supplement. I'm saying start reading. Wikipedia is a great place to start. Malabsorption, for a lot of people (my family included) is caused by eating the wrong foods. (The , homemade, whole wheat bread I was feeding my family turned out to be absolutely toxic for us.)
Good luck! My fingers are crossed for you that this will be your answer as it was for us.