So make the breath weapon d8/2 hd (total hd) of the creature. No max, once per day. That scales fairly well and is still at least somewhat useful through most levels. Plus the breath weapon is the only thing that really makes this guy a decent +1, might as well try to keep it without making him a worthless +2 
Edit: or d8/3 hd, 1/day per point of con bonus. That would be even cooler, weak but can be used more than once in a blue moon.

Edit: or d8/3 hd, 1/day per point of con bonus. That would be even cooler, weak but can be used more than once in a blue moon.
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