I dont know about Miralax. Does it push liquid through without being absorbed? I used Movicol, which does. Movicol was the only thing that worked. Perhaps ask the pharmacy if its a similar thing.

Do NOT add any fibre right now. Fibre is useful for maintaining regularity but right now, no.

Do not muck about with this. Right now everything is compacting up. Soon it will be compacted up to his stomach. He wont be able to medicate or hydrate orally, he will vomit everything that goes down. He'll be at the ER getting an xray to confirm that its all impacted, and need to be hydrated through a drip. The bowel will distend and be very distressing and uncomfortable for days.

They will stick him full of bowel prep to flush it out, which is basically the same as Movicol. Its a liquid that stays liquid from top to bottom, flushing everything in between.

It was 7 days before I went to the ER, followed by the worst 5 days of my life. Please don't let it get that bad. Talk to the pharmacist about whether Miralax will do the job. Senna certainly won't. All it does is soften the stool, it wont move it. You can talk to the doctors and nurses, but I doubt they will tell you much different. There is no magic pill once you get to hospital, it just needs to be flushed out. And it takes days to do that, very uncomfortable days.