So my MIL gave my son oatmeal Saturday night after I told her it makes him extremely constipated and…

So my MIL gave my son oatmeal Saturday night after I told her it makes him extremely constipated and now it’s Monday and he still hasn’t pooped. He was trying all day yesterday and couldn’t and now this morning he is in so much pain trying to poop I feel so bad. Idk what to do to help him right now. Does anyone have any tips?