Poop issues - allergy?

Every couple of weeks if not sooner my son goes through a stage of having a hard time pooping, it'll be sandy texture and it'll make his butt raw. He'll poop like every 20 minutes a little bit so i'm having to change him often; which is fine but he's screaming and thrashing because it hurts.
It's almost like he's constipated because he pushes and has a difficult time but only a little comes out. But he acts like the whole process of pooping is painful, like he'll shake / cry during it.

I don't understand why he keeps doing this; his diet doesn't change, he has plenty of fiber in his diet etc.
I'm beginning to wonder if he has either a gluten or dairy allergy. Idk if I should make him an appt with his pedi or what?! But i need to do something, i hate seeing him in so much pain  

My son has these issues. He has food allergies and is going to be tested soon for stomach issues.