Well, he might have gone *too* far or he might have been too hard. Anal sex may be very dangerous if it is done with no caution. Was he going too fast? Or too deep?
If you start feeling uncomfortable, then tell. Remember that the anus and the colon is not that big (may get to 7'' or 8'' deep in a full grown adult) so if he is really that huge, it would be better to not go all the way in. At the end, he will enjoy it more if you're enjoying with him.
-Jean.