I didn't have a boyfriend but my father in law did marathons up until a few years ago, including the Boston Marathon several times. He was never lethargic or grumpy. Quite the opposite. He was upbeat and it was like a drug. He was exhilarated. It sounds like your boyfriend isn't paying good attention to nutrition. When father in law was training he'd be eating several thousand calories a day. He was quite small too, very thin and 5'5"