First off, I worked at a drug store in high school and I remember that we had one customer for whom we special ordered goat's milk because the formulas of the time did not agree with their baby. This was the 1970's, so there was not the variety of formula there is today.
With my granddaughter, she ended up on soy formula because she tolerated it best.
With my daughters, I breastfed. The first one I had good success pumping at work to have milk to take to the sitter. If she needed more, she took a bottle of formula without problem.
With the second child,I had a different job and was not successful pumping at work, so I'd nurse her in the morning and night and on the weekends and then supplement with formula. She got so that she preferred the formula and weaned herself at 6 months whereas the older daughter weaned herself at 12 months.
Somehow mixing the formula and breast milk seems sacroligous to me. My first babysitter with my first child was trained as a live-in nurse for first time moms. She would stay with the family for 4 to 6 weeks to help them ease into life with a baby. That was in the 60's when such services were available to wealthy women. As part of her training, she said that they had to do a taste test of milk and several formulas. I don't think I ever did that, but there is a distinctive difference in the taste. I know there's a real difference in the smell and the texture and color.
I was watching some old episodes of the old George Burns and Gracie Allen show which was sponsored by Carnation evaporated milk. I had forgotten that before the days of canned and powdered formula, mothers made their own formula using evaporated milk.
If you haven't already, you might try the baby of a variety of different formula one at a time for a week or so at a stretch and see which one he prefers.
Weaning my girls by nursing when and while I could and gradually replacing their feedings with formula and juices and solid foods worked well. My supply decreased gradually and it gave me time to adjust to the emotions of moving past a wonderful and satisfying experience.