Some ideas...
A teleconverter is not going to be particularly useful on the 100-400 lens. I'm afraid your next step for more reach is going to be an expensive one... 500/4 IS. That's a fabulous lens, but expensive. (Sigma makes a 500/4.5 which is a little less expensive, but not a lot and it doesn't have IS, which is very useful on any 500mm).
Any teleconverter on the 100-400 with Rebel cameras will make it very difficult or impossible to auto focus (the Canon teleconverters won't even try), especially a 2X. It will almost surely be manual focus only at the long end of the 100-400 (f5.6) which is where you'd be most inclined to a teleconverter. Some of the third party 1.4X teleconverters might still try to auto focus, but the viewfinder is going to be dim, AF might only work in the most ideal conditions, and AF may slow way down or focus inaccurately, if at all.
Now, if using one of the 1D series cameras (or 1V or EOS-3 film cameras), a 1.4X should work on the 100-400, with the center focus point only. But assomeone already mentioned, I'm not sure image quality would be all that great with a zoom. (Canon 2x and 1.4X both work very well with prime lenses they are for - 300mm, 400mm 500mm, 600mm - just not so great with zooms.)
How about a macro lens for the little critters? Depends on if your husband likes to shoot small subjects. A 90mm, 100mm or 105mm macro can be a good compromise, also useful for portraits of people. There are Canon, Tokina, Tamron and Sigma macro lenses, all of which are pretty darned good.
The 24-70/2.8L zoom might be useful to him. It's got a macro range that gets in pretty close, too. Excellent image quality. Again, it sort of depends on what he likes to photography.
Does your husband like the wide angle of the 16-35, ever say he wished he had wider for landscapes and that sort of thing? A Tokina 12-24 or Canon 10-22 would give him that option.
I'm afraid that none of the above, short of the 500/4, will help very much with the distant wildlife you asked about. And, with the 500/4 you almost need a pack to carry it in and a special tripod & tripod head to use it with. (Can be handheld a little, and used on a monopod some of the time, or set on a beanbag.... more on these below.)
Tokina, Tamron, Sigma, Vivitar and others used to offer a 500mm f8 mirror lens that could be fitted to EOS cameras. However, as far as I know, no one is making one now and all will be manual focus only on any Rebel camera (might not have even been made in auto focus, I don't know). Now, when I shot with manual focus, film cameras and traveled a lot with them, I used a Tamron SP 500/8 mirror lens quite a bit. It was a good lens that could be adapted to most cameras (interchangeable mount, although the EOS Adaptall-2 mount is quite hard to find). Mainly, it was compact and light weight. But, manual focus is difficult with any modern Rebel and would especially be so with an f8 lens. Still, it's an idea. You'd want to talk it over with your husband though, before going this route.
Top quality polarizing filters for the 16-35 (82mm) and 100-400 (77mm) would be useful, if he doesn't already have them. Look for Hoya Pro MC, Heliopan SH-PMC or B+W MRC or B+W Kaesemann for the best quality. Polarizing is probably more useful on the 100-400, it can overly saturate skies on a wide lens like the 16-35.
The only other filter I'd recommend, in both sizes, would be a UV (B+W calls it a KR 1.5 I think). Look for the MC, SH-PMC and MRC designations in those brands, again, for the best quality.
A lot of people put UV filters on their lenses all the time to "protect" them. I don't and personally think that's rather silly. There are times some protection is useful, but not all the time. However, a UV filter can also help images be more saturated in hazy and dusty situations, which might be the case.
Kenko extension tube set might be handy for use with the 100-400 if shooting small birds relatively close (I don't have that lens, so am not really sure how close it can get on its own, without extension tubes).
Someone mentioned a monopod, useful even with IS lens like the 100-400. A tripod is useful too, but may be a hassle in some situations.
A beanbag is another great support, especially if shooting from vehicles. You can pack and take one empty and just go to a market, but and fill it with red beans (or rice or similar) when you arrive. Put a large plastic bag inside the bean bag, to hold the beans (or whatever). When you are leaving give the beans to someone who can use them and pack away the empty bean bag again. Easy to pack and light weight.
Does he have vertical battery grips on his cameras? These double the battery capacity of the cameras, so he can keep shooting a lot longer. They also help the smaller Rebel cameras feel better balanced with larger lenses like the 16-35 and 100-400. And, vertically oriented shooting is made easier with duplicated controls on the side (shutter button, etc.). I have battery grips on all my user cameras.
Any use for a flash? The Rebel built-in flashes are pretty wimpy. A 580EX II (bigger, but more powerful) or 430EX (smaller, less powerful) might come in handy. Best used on a flash bracket, mounted off to one side, which requires an off-camera show cord (OCSC2 or OCSC3). And, a flash extender can be very helpful with a long lens like the 100-400 and wildlife (Better Beamer is one brand, Flash Xtender is another... You need to buy one to fit a specific flash, though.)
but gives you an idea of the place
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