One of the benefit I've noticed to Underworks binders (or at least the double front compression shirt) is that it's comfortable enough to wear for several days. Most transguys say not to bind for several days, but I've done this and have yet to face any complications and it does flatten my chest out significantly; the only uncomfortable thing I've noticed about this binder is it occasionally will slide around and roll up in areas, which is not usually too big a deal, but occasionally it's enough to make it more obvious that what is underneath my shirt isn't actually a nice set of pecs.
My first binder was a lesloveboat binder, and I can't say I remember it binding much better than my Underworks binder...if anything, I remember it binding worse. I could have ordered the wrong size, but even then, I never felt even remotely flat with this binder on and if someone were to touch my chest by accident, it would have definitely felt like there was significant breast tissue there. The color was a problem, but that was more of personal mistake. I chose to get a black binder, but soon realized I could not wear it with any of my lighter t-shirts without looking entirely ridiculous. Lastly, the binder I had was uncomfortable, because of the Velcro. I know that not all binders by this company are Velcro binders, but I seriously have a bad experience with binders that are made this way, just because readjusting my binder often puts me at risk for a pair of nicely scratched up knuckles (and often underarms from a day of wearing one). I don't know if anyone else likes these types of binders, but they are certainly not for me.
I recently ordered a v-neck binder from Peecock Products without realizing that it used some Velcro on the side and also ordered the wrong color (black -.-), but I have to say that it actually bound me really well...in fact, even better than my Underworks binder. But it's still uncomfortable and I had to take it off after a few hours of wearing it and this wasn't because I was all "help, I can't breathe", but because my skin was getting irritated from the Velcro that, without exaggeration, had the texture of extremely coarse grit sandpaper.
In my opinion, if you want your boyfriend to feel comfortable, I'd get him a binder that fits like the double front compression shirt, because it's design resembles how an undershirt would look and therefore there is less concern about the outline or the binder showing up under a t-shirt and looking extremely "off". This is my opinion anyway. >_<
