wow. that is a tough question. with a history of milk allergy in the family, i would be very hesitant to try a cow milk based formula, and i would also be hesitant to give a soy-based formula to a boy because of the (as yet mostly unsubstantiated but entirely plausible) possible estrogenic effects.
i will share a bit of our story, though.
we discovered milk supply issues when our son was about 4.5 months old. pumping to help boost supply didn't appear to work, herbs and tinctures may have helped minimally, oatmeal may have helped minimally, domperidone (in addition to being expensive) may have helped minimally. we supplemented at the breast with my puny amounts of pumped milk, donated milk from a mama friend and occasionally with bottles of fortified goat's milk (fish oils, sunflower seed/flax seed oil combo, iron, folic acid). we also introduced solids.
at around age 9 months, our son became much more interested in eating solid foods. about 2 weeks later, he began eating as much for breakfast as i would (a big bowl of oatmeal AND a piece of fruit after nursing less than an hour prior). i thought i would try to not give him supplemental milk at the breast for a few days and see what his response was. he did not appear to miss it and continued eating solids ravenously. he was and still is smaller for his age, and he actually gained the most weight during the last few months of his first year while reducing his breastfeeding. i don't know that i would recommend anyone do what we did, but i do know that i followed his lead and this is how we walked the journey together. he breastfed until he was 18.5 months old, and i became unexpectedly pregnant less than a week later. i think he still remembers his "ma milk" because every now and then he sees me getting dressed and asks for some and gives them one suck and asks for the other and then pulls my shirt down. i am open to him nursing when this new babe comes along if he wants to, although it makes me nervous to think about possible supply issues again and nursing two children.