That he yet dreams of the lions is symbolic that Santiago looks to his past for strength and inspiration, rather than focusing on the future which is, perhaps, bleaker. The lion is often symbolic of courage, so Santiago's dreams may signify that he finds strength in his dreams.
The lions seem to symbolize the old man's youth. They might be read as a symbol of virility. Certainly, the lions are associated with a formative time in the man's life when he was seeing new things; living a different life. He was young then.
Also, we might read the lions as a sign of an alternative life. They are part of a fantasy for the old man, a favorite fantasy, not unlike baseball. The lions live on land in a different country on a different continent. It's hard to think of a creature more representative of a different life for the old man, who has spent his life at sea, growing extremely familiar (and even familial) with the creatures that inhabit it.
The Old Man and the Sea is the story of an epic struggle between an old, seasoned fisherman and the greatest catch of his life.