We live at 7200 ft in Colorado. I took DS up (driving) to the top of Mt. Evans when he was 10 weeks old. We did not go gradually, we drove, and I watched him very closely. He seemed a little spacey to me, but handled it well other than that. He's been up to 11k-12k feet several times since then (we haven't climbed any 14ers with him), and he's been just fine.
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I think the same rules for adults apply: go slow (you plan to), climb high and sleep low (you plan to), make sure baby is hydrated - if you need a water break, baby needs a water/nurse/bottle break, and of course be attentive to his cues. Oh, and remember to pay close attention to sun protection for your babe - at 14k ft, you'll be nearly 2 miles closer to the sun than at home, in much thinner protective atmosphere. Consider sunscreen even if you don't normally do it, and reapply often.
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I love hiking with the Ergo - it works very well. We also have a Deuter Kid II frame that DH wears for longer hikes... Your DH might consider if he'd prefer a frame. He might be warm in the Ergo with the baby right on him. I'd definitely prefer the Ergo for winter hiking.
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Do you do much hiking with him at home? I think the biggest thing I'd be worried about is the logistics of a long trip, especially diapers.   Have you been camping with baby? My DS has taken to camping very well after 5 nights so far. If you haven't, do some practice nights in your backyard, or in the frontcountry near your home so you can get some systems down.
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I'd absolutely take an 8-9 month old on a 14er. I hope to take DS out to the mountains a bunch more next summer - he'll be 13-17 months. Get them into the outdoors early and often! Best of luck!
