Take him off soy immediately. When it comes to infants they will let you know if they have a problem with a food. Spitting up alot, throwing up, fussiness, gas, diarrhea, constipation, change in sleep patterns, and crakiness are all signs that your child can not process a certain food. If these symptoms occur within 1-14 days of trying a new food and get worse, then it means that you need to change the food. This is also true when starting on solid foods. Also make sure to burp him more frequently with formula. With breastmilk, my son didn't need much help burping, however once he went to straight formula, he needed the help. If the regular formula causes spitting up after the 4th day, then I would try Similac Sensitive. If it is the same with that, then your next step and last step would be go to Similac Alimentum.
Tip - when giving formula try these steps if you see problems. 1. If using pre-mixed formula, buy a can and mix yourself. The pre-mix can be too strong. 2. If mixing yourself, make sure to boil, then cool the water and make sure the water if filtered or bottled. 3. If still having problems try putting a couple of teaspoons more of water than it mix says to add.