Do not give him an alternative to healthy food. Hes only testing you and by you giving him cookies and sweetened yogurt, hes winning. So here's what you do..... offer him only healthy foods (that he has liked in the past), if he doesnt eat it, cover it up and save it for later. When he complains that he is hungry, offer him the same food. He will more than likely throw a fit, so be prepared for that, but dont give in. Dont give him anything else until he eats the healthy food. Once he sees that you are not going to give in, he will eat. Dont worry about starving him, its human instinct to eat when you are hungry, so eventually he will eat it. He may have one or two nights that he goes to bed hungry, but thats not going to kill him. Just stick to your guns and he will give in.