Maybe your older child is jealous of the attention the baby is getting at feeding times and is acting out to get your attention on him. Are you nursing? Did you nurse your older child too? My friend had a 3 year old she nursed till about 18 months. He would get really upset when his sister nursed. Once she asked him if he didn't like when the baby nursed. He said he did not. She asked if he remembers nursing, and he said he did and then started crying. She couldn't believe he remembered it after so long and he was obviously upset that his sister was getting that kind of attention.
Maybe when you feed the baby you can snuggle with him too and read him some books, that way he feels close to you still.
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