I need help with my 5 year olds behavior

Im literally so desperate for some encouraging words or anything really. Im 4 months pregnant and i have a 5 year old little boy. He is my entire freaking world. Lately though, he has been absolutely so defiant. I am currently bawling my eyes out in my room while he is in timeout in his. Its like everything i say he doesnt listen to. I tell him dont jump on mommas belly you could hurt the baby , and he does it anyways. I cant figure this out. I just want him back to the sweet little boy i know. I feel like such a bad mom. I think he may have adhd or austism so i try to keep that in mind but i cant let him walk all over me.