Coincidence or something else?

DS was on soy formula and doing just fine. Ever since we have switched from soy formula to soy milk his diapers have been disgusting. We thought that maybe it was the soy milk, so we tried almond milk as well with the same results. DH thinks we should put him back on the formula until we go see the allergist again at his 15 mo checkup. Anyone else's LO have difficulties switching from soy formula to another milk sub? Do you think DH is right and we should put him back on formula? We have even tried the toddler soy formula which was better, but still not as good as things were on just plain old soy formula. Any guesses what would be causing the problem?