My son has been on soy formula for over a month now, and has been very constipated, now his stool has changed and it's very hard balls and it black?

Constipation: Soy formulas are usually more constipating that others. , Rapid formula changes is best avoided (give a 2 week trial). A common trial sequence is: cows milk based, lactose-free, soy, elemental. In the meantime, take a rectal temp to stimulate the anus to relax. Blood, vomiting, poor feeding & irritability are emergencies. Lactulose is a well tolerated stool softener for babies (Rx)