DS is 8.5m and seems to be doing fine in pretty much every respect. He sits up well, is getting on with BLW, squeals and does a bit of babbling, sleeps ok(ish) - in short everything seems normal. However, when he's upright his head will often (and I mean several times a day) kind of drop forward and to one side. It seems to be involuntary - I can't tell whether it's because he's tired (it doesn't happen at predictable times) or whether it's some kind of involuntary spasm that forces his head down and to the side. It looks as though he's sort of 'peering' that way - always to the left I think. I veer from thinking I will definitely take him to the GP to thinking he'll probably grow out of it, as he seems to have grown into it. It has been going on for a good couple of months now - ever since he could sit up I think. WWYD?
Take him to the GP, ASAP, this could be some sort of absence seizure. This is not typical behaviour, don't wait for him to grow out of it - if there is a problem, it needs diagnosing sooner rather than later.
I'm sorry if this sounds harsh or alarming but there are several red flags in your post - this is frequent, appears involuntary, and his eyes going distant (Peering off tho the side), and you suggest it's got worse over the last 2 months ("he's grown into it"). IMO this is URGENT.