Just make sure he doesn't get his hopes up an unreasonable amount. I was pretty crushed when I first got my binder because I thought I'd magically look 100% flat, and that's just not the case. There's still the tissue there, it doesn't just vanish.  They'll give you a masculine shaped chest, which I'm guessing this shirt you're buying will help do, but they won't totally flatten you; not that males with any form of pecs are totally flat anyway. Just a warning to help stop him getting disheartened.

Thanks, I think he knows it won't make a huge difference (but then he's fairly guy-shaped anyway). It's more symbolic, but that's just as important.