Feeding my son

My son is 4 1/2 months old and we’ll be doing baby led weaning once he’s 6 months old, so I won’t be giving him anything until then. Every time we go to my boyfriends parents house they try to feed him and its driving me crazy. Today my boyfriends mom tried to give him watermelon and I literally put my hand between his mouth and the watermelon. They have lost babysitting privileges since she gave him water when he was just under 4 months old. But every time we go there I’m always having to worry about him and stay around them to make sure they’re not trying to feed him. Also his dad joked around today by putting sausage close to his mouth when he knows he will try to reach for it. Just wanted to vent because I’m so angry/sad about it since they never listen to me and my boyfriend doesn’t care since his mom did all this and he’s fine.