Hi Rachie,
I'm a Mom with 2 girls & and 11. When my youngest was a baby she suffered from constipation. One time I had to actually pull a hard piece out for her. The doctor recommended pear nectur in her bottle and glycerine suppositories. Since your son is 2 and a half then you could make sure he eating good amounts of fruit and vegetables. Bananas can also be constipating. Avoid apple juice as it can be constipating. For the rash I'm assuming he's still in diapers right. Keep him as dry and clean as possible. Also your doctor can prescribe a medicated cream. My youngest had a bad rash once where she actually had little raw spots. You can even let him sleep without anything on his bottom at night (just protect the mattress) and that will let the air get to it. I've heard of children who hold their bowel movements. It's most common during toilet training.