My maltese boy is three months today and 3 pounds. (His mom is 5). Any idea of his full grown size???

I would also like to know at what age is a maltese full grown and if the Cesar (small animal food) in the puppy variety would be good for him. I was so stupid as to buy him Purina One and now I'm not sure. Do you think alternating two kinds of food will be good to prevent him from getting bored?