Maybe too late to reply for your boyfriend's birthday, but I personally find phones and computer a risky thing to surprise someone with. They may have their sights on such a device themselves and if you then come with a phone he doesn't want, he would probably feel a bit angry, because okay, it's cute you got him a phone and all, but it would be rude for him to then buy the phone he wants and puts your gifted phone in the closet as a reserve, never to be used. Not to mention the money wasted.
I recommend talking with him about the matter first. It's no longer going to be a surprise, but better the perfect gift than a louse surprise. If he has sights on a phone that's just too expensive, hear if anybody wants to chip in. Then they have their present too and you don't pay the whole price.