S.H.
You listed all possible possibilities... except: "Growth spurt." Typically, at 6 months, babies have a growth spurt.. and they get hungrier, need to feed more often... because their intake needs INCREASES and needs to keep up with their growth.
Babies/kids will wake more during growth spurts. Per our Pediatrician, for the 1st year of life, a baby needs to be fed/nursed ON demand 24/7, and it is their PRIMARY source of nutrition... NOT solids, and NOT other liquids.
I would, nurse him when he wakes... I suspect, he had a growth spurt at 6 Months... and is still trying to catch up with having more intake to meet his current needs.
Growth spurts occurs about every 3 months. So the next one will be at about 9 months.
AND, also, it can be that he is hitting milestones, and/or, normal developmental changes and cognitive changes, and separation anxiety.
Many times, sleep aberrations are due to MORE than one single thing at a time.
Chalk it up to normal developmental changes and growth spurt.
No baby sleeps the same way, nor the same pattern all their life. It changes, per their age set and developmental/biological changes.
It will return to normalcy. One of MANY phases. More will arise at different age junctures.
Also, not all teething episodes are the same... molars for example causes more pain/discomfort. So, you need to go according to that and how it affects them.
Nothing is static, with a baby. And, sometimes they wake just because, and there is no real reason, or "logical" reason for it.
A great book is "What To Expect The First Year", which you can find at any bookstore or online like at Amazon.
All the best,
Susan