I'm stumped by this one. I'm 6'4½ and I wear size Medium to Large in various brands, and even though I wear my pants lower than most people on this forum I've never had a problem with a single shirt ever being too short for me, except H&M t-shirts and that was only after I accidentally shrank them in the drier. In fact, I find most shirts annoyingly long, such that I think they look weird worn untucked over jeans.
I've had a few Lacoste polos, I sold them so I can't check, but they were size 6 I think (whatever corresponds to Large, roughly speaking) and they were a fine length for me, actually maybe an inch longer than ideal. Penguin and Ben Sherman make polos the perfect length for my taste, which come just down to the level at which I wear my belt/waistband.
Etro makes very long dress shirts in my experience, although they are prohibitively expensive at retail price. Maybe take your BF to Holt's and try on some Etro shirts, and then if they fit well, look for them on eBay? (there are lots around the $80 range, which is quite a bargain for that brand!) Perry Ellis also makes shirts that are quite long, and they retail for much more reasonable prices (~$70 MSRP) but of course they don't compare favourably to the likes of Etro. I also find Hilfiger shirts are often quite long, but they are usually very baggy and if your boyfriend is exceedingly thin then even a Small by Tommy might be too baggy for him.
Also try French Connection (and maybe H&M, which is right nearby if you shop on Bloor St., just not their undershirts! hehe) because their clothes aren't especially long but I find they are tailored for relatively thin individuals, so whatever is long enough might have just the right girth for your boyfriend.