To try and answer the question about the vomiting at the bedtime feed:
There are a few things that come to mind...
He could be swallowing quite alot of air during the feed (vedry common for refluxers) and doing a wet burp which causes the vomit, however from your description it sounds as though this isn't the case. How does he feed? Is he fussing and on and off the boo at all? Is it aggressive ferocious feeding?
How is your supply at this time of night on that second breast, is it still quite full and the flow fast?
My LO has vomited like that when he has absolutely gorged himself and not had enough burps - it happened when he first started going 4 hours without a feed instead of 3 hours - he was soo hungry that he ate and swallowed alot of air and then burped it all back up in one big vomit. As a precaution, it can't hurt to do extra burping.
How long before the 5pm feed has he fed? If it is less than 3 hours maybe he is getting very full. Do you have a big supply in general? Maybe he doesn't need other side? I have a very big supply and fouind that my little refluxer could only cope feeding one sided even at bedtime
It would help if you could post your feeding routine for the day. Maybe the build up of feeds - causes him to feel stuffed.
The only other thing I can think of which is a long shot could be that he he is reacting to something that you have eaten between noon and bedtime. Do you have a milk or soy drink at all daily during these times?
hugs HTH