So he's what about a year old now ? Almost a juvenile. You should be introducing veggies to him , if not taking them keep offering them to him and will take. What I do with the young ones is feed the veggies first before the protein. Here's a link for good veggies :
http://www.redearslider.com/plants.html Also a good variety of all turtle food is good for a balanced diet. Omega One is the freshest and the best. Do not let the begging get to you , they only get better at begging with age. Better to underfeed than overfeed ! Use to 2/3's rule ! For protein imagine if his head was hollow "not the neck to" what could fill 2/3's the head size is all a day. Not at each feeding. The veggies say red lettuce the size of his shell is good ! If you want to feed twice a day just divide that above amount in half at each feeding.
If he don't eat the veggies leave them in the tank he may eat it during the day. If not remove uneaten veggies at night and try the next day. By the time he's an adult he will be on 70% veggies and 30% protein to be healthy.