Sounds to me like your husband is trying to give your son what he always dreamed he wanted. Food was love in his home growing up, so this is familiar to him. He's combining treats and doing those "fun" things all together. It sounds like he's trying to be a good dad. However, you're right. Giving to your son like that isn't healthy at all.
You shouldn't try to compete with your husband with giving your son treats. I would find something different and special that you can do with your son that doesn't involve food. I have 3 boys and when they were little we used to do projects together. At Michaels you can buy cheap kits and do those things together - ie...Shrinky Dinks. My kids love those. Also, I suggest you go through some magazines together, like FamilyFun and look for some fun project to do together. Last month they suggested collecting rocks and painting them. Another idea that all 4 year olds love to do is to cook with you. Yes, it is food, but it's the idea of doing it together. My kids love it when I involve them with cooking our meals or baking something special.
Also, while you're doing this you should talk to your son about proper nutrition and how eating sugary treats are good occasionaly, but bad for us if it's all the time.
As far as your husband goes, you need to sit down and have a talk with him. Maybe since it brings him so much joy to give your son a treat, you can ask him to put limits on it, like don't offer it until after dinner or finding the right time that you feel comfortable with it. Also, maybe you could ask him not to do it everyday, but only on weekends..
Sitting in the front seat without a booster is illegal, plain and simple. Do some research about traumas that have happened to children who aren't properly buckled and share it with your husband. Maybe it will help him understand your concerns.
I would just ask your husband to put limits on his type of attention for son. Like watching cartoons for only 1 hour at a time.
Hope this helps.