Congrats to him for taking steps to be more comfortable, and congrats to you for being an awesome and supportive partner. He is damned lucky to have you.
First off, it's important to remember that the way you (him, you, anyone) dress should be the way that makes you comfortable--not necessarily the way that you think will get other people to see you a certain way. I don't bind tight every day and I don't wear all men's clothing every day. What matters is that I am confident in my own identity. If other people don't get it, that's their problem, not mine.
That being said, I got some fairly large lumps to cover up, and I do prefer to minimize the femininity of my body. What I usually do is layer up. For casual wear, I get some comfy men's shorts or jeans that are loose enough that they don't look tight in the thighs (like women's jeans), then a t-shirt and a button-up shirt (like an oxford) open over the t-shirt. You can also layer an undershirt below the t-shirt to smooth things over even more. I usually wear loose shirts because I lost weight and everything I own is now voluminous, but when I wear a smaller, closer-fitting tee, I find that tucking it in helps flatten the look of my chest. Leaving the button-up shirt loose and untucked helps hide curves at the hips and waist as well as the chest. In addition, vests are very nice for hiding curves. I love a pinstripe vest over a close-fitting tee shirt and skinny jeans personally, but I go for a more androgynous look than a lot of guys are comfortable with.
Less casual wear can be dress shirts or a nice polo buttoned up over a binder and undershirt. The undershirt smooths down curves. Going up one more notch, there's always the suit (which can be dressed up or down depending on the shirt, pants, tie, lack of tie, or even pocket square and ascot you choose). Go for single-breasted and try to find a style and cut that hangs well. I'm a fan of the three-piece myself, especially with skinny jeans instead of slacks and a casual shirt under the vest, since I love vests and they add another layer of chest-hiding goodness, but they aren't always appropriate. Here is a good guide for choosing suiting to minimize the feminine figure:
http://www.ftmguide.org/suits.html (that whole site is worth reading, btw).
As for accessories, I would go for baseball-style hats, trilby/derby type hats, knit hats, men's style belts (webbing or leather, with a masculine buckle or D-rings), and large watches/wide leather wristbands.
Shoes are up to you--converse, combat boots, oxfords, running shoes, and nice leather flip-flops are all fairly manly and "in."
I get all of what I wear from thrift and consignment stores (well, except the binders). It's a great way to try out styles for cheap. I like to think of myself as the king of the $10 outfit.
