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Use a baby gate that requires installation into the wall, or move a heavy piece of furniture in front of the fireplace temporarily. Don't be afraid to tell him "NO" extremely firmly and follow up with pulling him away. Once he's old enough to understand time-outs, start using them in conjunction with going near the fireplace. Eventually he'll get the idea, but absolutely NEVER leave him alone in the same room as the fireplace if you've got a fire going, until he's much older.
With my daughter I told her the stove was HOT and it would hurt, we used a firm voice. She has never tried to touch it. For the most part if we tell her something is going to hurt her she listens. The exception is the cat, probably because he is so good 99% of the time. They do make gates that wrap around. We are moving into a house that has a fireplace so we will see how that goes. My son is 5 months so we will have to teach him. I burned my hand on a wood stove when I was 1 1/2, I bent down to pet the dog next to it and not thinking put my hand on it for balance. I have no scars but I guess it was really bad.
My first suggestion is to not use the fireplace. We just moved into a house that has one but we won't be using it because they are so inefficient for heating. If you do plan on using it while your son is awake you'll need to buy a fireplace guard, which is essentially a big, usually metal, baby gate type thing that stands a foot or two in front of the fireplace and wraps around it completely.