Why doesn't he trust anymore?

So last night my boyfriend and I had a huge fight over how we deal with our family, specifically when they go against our parenting wishes. He thinks I'm too strict on his family and not my own because my mom did something he didn't necessarily agree with. I talked to my mom about it and agreed not to do it anymore. He wanted consequences for her actions since his dad cannot keep our son overnight anymore. His dad gave our son three to four ounces of water like four to five times a day over the course of a weekend. At the time our son was only two months and it lead to him being in severe stomach pain due to constipation. I had take our son to the doctor and pediatrician told me to switch formula because the water causes some digestive problems. He even lost weight because of it. My boyfriend knows this and still told me he doesn't trust me anymore because he feels I don't tell him when things happen and that my mom will keep doing what she wants when she keeps our eight month ols son for the weekend once a month. I was completely shocked,I can't fathom why he all of the sudden doesn't trust me. I've done nothing wrong, hell I barely leave the house since I'm SAHM. So is he just mad my mom's actions (letting the baby eat pizza crust) wasn't really a big deal to me, or is he because I do not 100% trust his dad to be alone with the baby still, or does he really not trust me?

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