I am looking at the shore excursions book that HAL sent with the docs. The only mention of weight is if you're a child, you must weigh at least 40 lbs. I would think your husband would have to be fit, because of the rowing for 1 1/2 - 2 hrs (geez, I get exhausted just thinking about that!)
In Juneau, the flightseeing tour says that anyone weighing over 250 lbs will be charged a surcharge. I would imagine that there might be a seatbelt extension provided, which could prove to be embarrassing, depending on the person's attitude toward this.
Hope that helps. If you need more info that's not posted online (re: excursions) let me know.
Felicia