My daughter had similar problems - bleeding anus from the 'rocks' she pooped out while screaming!
It was the iron fortification of the rice cereal. The cause/solution are similar to adults : iron supplement=constipation -> more fiber, more liquid, stool softener
Since you are a celiac , and he is having problems I would put him on the GF diet too and have a blood test for him.
My pediatrician suggested pears in addition to prunes for fiber.
This was 7 yrs ago and I was able to buy non fortified cereal, along with lots of vegetables (fiber) and nursing fixed the constipation. my second daughter is better but I have to limit the cereal I give her too.
I developed a sensitivity to wheat after my 1st daughter was born, but all the tests are negative. My second is very sensitive to onions/garlic, but again the blood tests came up negative.
My son has had a constipation problem for months and months too. He is 9 months old now. The only thing that has worked for me, and trust me I have tried it ALL. I just now finally got him to drink juice from a sippy cup. He has about 4 oz a day not all at once, but rather here and there through out the day. I rotate pear just with a mixed juice about every other day. This is the ONLY thing that has worked for me! Some days are better then others, but it is a process that we are trying!
Good Luck, I know how frustrating it is!!