We've had constipation issues like yours for a few years now. My daughter is picky so I couldn't really try the GFCF diet. We were giving her miralax daily according to her pediatricians advice. She still had days when she'd hold it in for close to a week and then bleed when she finally did. She also ended up many times with encopresis. She's always drank a lot of water since she was very little, so much so that I once took her to the pediatrician who did some tests to see if there was something wrong. So, fluid intake was not the issue here.
The past one month, I've been doing a lot of research and came up with the following concoction.
1) 2 to 4 scoops of benefiber in her drink (usually coconut water or milk)
2) Flaxseed powder, about 1 to 2 tablespoon mixed in her food (I read about this on MAT)
3) Digestive enzyme for kids that I bought from amazon
4) Probiotics- Theres only one brand she'll take and I pray everyday they do not stop making it.
5) Mango slices- about 5 to 10 bite sizes ( We're doing feeding therapy at ABA for her picky eating. She now eats strawberries, we're now doing mangoes cause it has a lot of fiber)
6) Miralax daily continued, but I'm now changing to every other day and plan to wean her off of this first.

She's been having at least 2 BMs a day for the past 2 weeks since I started giving her the above. At least one of them is a major blowout. The first few days she had like 7 to 8 BMs, it was terrible but great!! 😄I'm trying to cut back on some of the things to get a good regular schedule for her. Needless to say, I'm more than thrilled that this is happening, but feel a bit bad cause her school teachers and therapists have been the ones to have to clean her up. They've been really nice about it, but I still feel bad. They do understand that this has to happen before she can be fully potty trained. I sent them a big box of gloves to use. She's been potty trained for #1 for over a year now but not for #2 because of the unpredictable poop schedule she had and fear of sitting in potty to poop. I'm hoping we will finally have her fully potty trained this summer before she starts kindergarten.

So sorry to hear your and his struggles. We struggled with gluten sensitivity but that’s opposite of what you are facing. I would give those new children’s probiotics to him and slow down on the enimas. He may just be Anal Retentive. Lots of water and exercise too.
All of this inability to use the bathroom (#2) affects his other behaviors. A small amount of melatonin at night (2 mg) will help the stimming as does chewing gum (if you can teach him not to swallow it)
Floor time helps with communication. It takes a lot of effort love and patience to spend time connecting with him. Also, make him a ‘nest’ in the corner of one of your rooms. Put soft pillows, small bean bag, different textures for him to find comfort (sensory) and have a place for him to experience and engage with him in these arenas...add books and toys that are also stimulating. Hope this helps!

How old is your son? My nonASD nephew had the same issue at 5 through 8. Apparently its common in boys that age. They get so constipated that they have leakage. He had to be given colace on a daily basis for about a year to clear up the issue and a high fiber diet to keep him regular after that.

The tests came back negative for my trio, too. A gluten reduced diet with a lot of raw fruits & veggies, fish, and limiting cheese helped my trio. Also probiotics and Epsom salt baths. The mini trampoline for jumping helped, too, before we got our bigger one.

My advice follow the child's lead!
let the child eat what the child wants to eat and do not push foods on the child.
Let them eat what they want.
That's what I do works great.