Cockers & aggression issues

[julescrew]

I have recently taken in an elderly client's 8 year old cocker spaniel, just under 60lbs. They fed him from their plates and wet dog food. He had little to no training & socialization and very little exercise. Since then he has moved in to a place with two kids, ages of 6 & 8, 2 cats, a reptile, & a rat. He has done suprisingly well. I walk him at least twice a day on a gentle leader. I have changed his feed to a commerical lite hard food. He is awesome with my kids, for the most part. He gets aggressive when he steals food, hard toys, & paper products and you try to take them away. When you go to correct him, he will show teeth & snap at you. He does not like for new people to pet him on his head. I'm wondering how do I go about breaking this bad & dangerous habit? He's not aggressive with the food his feed dish (the cats eat out of it with him). I make sure he knows I am the one in control (he no longer goes to the bathroom in the house) but this one issue I can't quite break. Please help!!! Thank you!