First, a good, wide neoprene binocular strap for carrying it. The strap that comes with the 15/18x50 IS is too narrow and hard to take the weight comfortably.
Secondly, a good set of rechargeable NiMH batteries (AA/RH6 size) with a charger and a little box for carrying the spare set. I use the Varta 2700 mAh Power Accu, but I have heard good things of Sanyo Eneloops also. These take care of the concern of running out of power.
Thirdly perhaps a lightweight monopod or a monopod walking stick. By this I mean a telescoping pole that has 1/4 inch thread at one end. The Canon has a tripod thread in the body, so you can thread a small monopod like this right onto the binocular. The advantage is that when you have the monopod at a lenght of some 15-30 inches, you can keep your hands down at roughly waist-level and view in comfort literally for hours, while regular hand-holding tires most arms in 5-10 minutes or less.
And fourthly, at leas if he plans to use them in inclement weather, an eyepiece rainguard would be good. Canon does not make a suitable one, so you would need to find a generic one that fits, and the Canon eyepieces are very large so many don't fit. I use a Fujinon rainguard made for their FMTR-SX range of individual-focus marine binoculars, but some others will probably fit as well.
Fifth, perhaps a high-quality microfiber cloth made for wiping coated eyeglasses and lenses.
The binocular comes with a lightweight bag which is okay for most purposes, but for better protection during travel a proper padded camera case or a gun case of a suitable size would be an improvement
Kimmo