My child has some behavior issues and I don’t know where to get help.

I have twin 4 yo boys and one 2 yo boy. The one twin and the two yo are easy toddlers with sweet personalities and what I would consider to be normal tantrums and difficulties. The other twin though is a mood swing machine. He might be on the spectrum and we work with our states headstart program and have an IFSP but they are not very helpful with issues we have at home. He doesn’t respond to the recommendations they give and he’s pretty easy at school so they don’t see what it is I have to deal with.

I live in a very rural area and my sons GP didn’t have any good recommendations so I’m hoping someone can help me ask for a specific referral. My son is so smart sometimes it’s like living with a teenager but he cannot handle his emotions at all. He won’t take a no and he has this idea of fair that if broken turns into a complete meltdown. He completely shuts down and you cannot go near him. He will hit and throw things and he doesn’t want you to talk to him or touch him.

The other major issue we’ve had is using the toilet. He HATES having to pee or poop to the point where he will hold it for hours and hours in absolute agony. I didn’t know someone could hold their pee this long and sometimes I just take his pants off and he will angrily pee on me. He says going to the bathroom is scary and he hates how it feels. He doesn’t have a uti and we give him Miralax so his poop is soft.

Who do you go to with these sorts of issues? Who diagnoses/treats ASD in children?