Laura's pretty much suggested what I would have!
At 5mo teething could definitely be a culprit - those buggers can cause problems long before they show themselves. In addition to teething gel (or instead of, my ds always hated it) you could try giving pain meds about 30m before a feed if possible.
I agree that the lack of interest in the wake up feed could be due to the night feed. Does he feed both sides? You could try limiting to one side, or limiting the time, or use a bottle of ebm and only offer 2-3oz. Often, when there is an early morning feed, the wake up feed becomes more of a top up, just enough to get your lo through to his next EASY feed.
I agree too that I wouldn't jump to a fast flow teat as it can cause problems with mixed feeding. You could try a variable flow one though (they're also called y cut I believe). That's what we use and they work quite well - the milk doesn't just flow out of them. They have to suck to get anything out, but the harder they suck the faster the milk flows, slow or lazy sucking and they don't get much. Just like bfing
hth