It is an unusual thing to say. It has no exact meaning - its meaning can only be metaphorical. To know what meaning the author intended, it would be necessary to read the surrounding context. By itself, it is a poor sentence. The adjective "lean" only describes the person ("he"). The sentence would be clearer if it stated more precisely what it is that is lean, and the possibilities are endless: "His desire to live had grown lean due to the memories he carried with him." "His memories had turned lean his ability to think clearly." "His fading memory became lean. He could only recall his childhood. He could remember nothing else."

Morning Bea. It either means he didn’t have many memories or he didn’t mention/talk about memories very much

While “lean” can have a negative meaning of “deficient in something useful”, it also has a positive meaning of “characterized by economy (as of style, expression, or operation)”. Think of a lean athlete who is all muscle and no fat, fit for purpose. I believe this is the connotation here. And reading the full passage backs this up, as the old man has a great number and variety of excellent memories. _____________________________ “My first afternoon in Denver I slept in Chad King's room … It was a hot high-plains afternoon in July. I would not have slept if it hadn't been for Chad King's father's invention. Chad King's father, a fine kind man, was in his seventies, old and feeble, thin and drawn-out, and telling stories with a slow, slow relish; good stories, too, about his boyhood on the North Dakota plains in the eighties … Later he became a country schoolteacher in the Oklahoma panhandle, and finally a businessman of many devices in Denver … He had invented a special air-conditioner … The result was perfect … Finally it was so cold I couldn't sleep, and I went downstairs. The old man asked me how his invention worked. I said it worked damned good, and I meant it within bounds. I liked the man. He was lean with memories. "I once made a spot remover that has since been copied by big firms in the East. I've been trying to collect on that for some years now. If I only had enough money to raise a decent lawyer… " But it was too late to raise a decent lawyer; and he sat in his house dejectedly.” — “On the Road”, (abridged) Jack Kerouac

Doesn't really make sense to me - I've never heard anything like that.