M.P.
My granddaughter was allergic to milk protein as a baby. The doctor suggested trying alumentum formula. It has no dairy.
If you haven't talked to his pediatrician that's the next step. You need to know what is causing these symptoms before trying to treat it.
It's important to give it for at least a week to know if it helps. Babies, just like kids and adults often don't like change and takes time to adjust to the flavor. As Diane B said, mix the soy formula with his previous formula gradually adding less previous formula. That makes the transition easier.
Does he often have a painful, gassy, upset stomach? If not, it may not be caused by the formula. Talk with his doctor.