My son is 3 and says his tummy is sore by his belly button and his tummy is swollen.. do i need to go to casualty?

Yes: If his tummy is swollen and he is not eating or passing stool he should be seen. Especially important is if his pain comes and goes in 15-20 minute intervals with or without vomiting or diarrhea he could have an early intussusception and this can be evaluated and . treated best when in the early stages. Although he may just be constipated, it is not possible to exclude the more severe conditions