Definately feed him once weekly. Weaned to small rats will be the best shot. Like the above post stated about the same size as his biggest part. I would go ahead and try and start him on FT also, you will want him eating FT when he is bigger for several reasons. If he takes FT I would even suggest getting FT guinie pigs. They have always worked well for me, I even have some balls that eat them. You said you didn't want him to get too big too fast, he is going to get big. Your main concern now is for him to be healthy.