2 yr old needs to go all day/diaper rash

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My son is always constipated and last time i put two suppositories on him and he still didn't poop what he had to. So its been 4 days he has been trying to go all day and just a tiny bit comes out and its not hard or diarrhea either but it IS liquidy but still solid? And sometimes its a little bit more. Since his diaper is dirty pretty much all day, he has gotten a really bad diaper rash that can't go away because like i said he's wet all the time. Soooo now idk if he doesn't go because it hurts or because something else is wrong but idk what it is or what could be. I'm taking him to the dr tmrw, although they never do shit so i guess we'll see what they say this time. And this wasn't the only time I've had to out two suppositories for him to go (for kids) but he did go that time but not this recent time.