Stressed out over my son I need advice

I feel like a horrible parent. Today we got up early and headed for the vet to get my dog groomed. I got days mixed up his appt was for tomorrow. And I thought a sausage biscuit from jack in the box sounded good, which is odd because I hate fast food. Well anyway my son said he wasn't hungry ant at the last moment was hungry. So we came home and ate. My son put half his food in the fridge and went to his room to play XBox. about 10 minutes later he came in and said I feel like I have to poop but I can't and my butt hurts. I told him go back sit down and put your feet on the stool and try again. Then he started screaming I got diarreah! Then said he was gonna take a shower. After that he said he felt better and asked to play his games and I said no. My emet mind took over and I started to panic thinking of the stomach flu. So I started asking him questions.

I asked what hurt? He said his butt hole.
I asked if his stomach hurt? He said no
I asked when the last time he used the bathroom? He didn't know

So my conclusion is he's constipated. So I told him no video games until he goes to the bathroom. I feel like a bad parent not knowing that he hasn't been going to the bathroom. So with his game threatened he drank 2 big glasses of water and thought he'd go instantly. He tried to go but he said only tiny bits came out. A couple hours later I gave him some miralax with apple juice. He still hasn't gone.

I just don't know what to do. I'm stressed out because I don't know what to do about constipation in kids. He's 8 and about 60lbs. What I've read is to call the doctor if its been 2 weeks since he's gone. I'm not sure if I should give him another dose of miralax? I know it's not harsh I can drink a whole bottle like nothing. But ive just never had problems with kids. I'm sure he hasn't been drinking enough fluids and he hasn't been that active lately.

Thanks. I just know first hand how bad it feels being constipated. My colon doesn't function so I have to take a lot of stuff to have a BM. Hopefully by tomorrow he will go. I'm really gonna have to pay attention more often to what he's eating and making sure he's getting enough fluids. I just feel bad about the whole situation. I try so hard for my children and I feel guilty when things go upside down on them.