How Much Should I Feed My German Shepherd?

I have 10 month old German Shepherd. He is 80 pounds and everyone thinks he looks skinny. I run my hands across his ribs and can't feel them unless I put pressure. I do not personally think he is thin but my parents do. Every time I go home from college they gripe at me that he is too thin. He is pretty active but not as active as most shepherds. He has a heart murmur and can't last long when he runs. There is a field where I live and I take him out and let him off leash to run and he can run around for 5 minutes tops so he is not overly active. I am currently feeding him Beneful twice a day and doing the amount on the back of the bag. I know Beneful is not the best dog food but right now it's all I can afford. I was feeding him Nutro but I am not working at the moment. Is 2 times a day for a German Shepherd enough or do I need to do 3 times or only once?