I go through cycles of thinking ds2 has traits that need investigating, then all seems ok, then back to worrying again. Ds1 is 8 and has recently been dx with AS, ds2 is 6 and in year 2.
Generally over the years I've been fairly sure that any of ds2's apparent traits or behaviours have come through having an elder brother with AS.
I know ds2 is sociable and more flexible than ds1 and generally an easier going child, but he does go through phases of being pretty rigid in his thinking and I'm pretty sure that of the three boys he plays with at school, two of them are most likely on the spectrum themselves. Its harder to tell with him, because he internalises a lot, whereas ds1 is very vocal and obvious if he's anxious or unhappy in any way.
I had taken the line, that as long as he's doing well academically, has lots of friends, is generally liked by the other children and has never been bullied, that even if he does have AS, he doesn't actually need any additional support, so I would watch and wait. I'm aware that not all children with AS need extra support, especially if they're very HF, it really depends how it affects them. As Marne said, in her dd1's case she hasn't really needed a lot of extra support as she copes well.
However, recently (since about December) he has seemed increasingly weak, debilitated, exhausted, oversensitive and tearful and is starting to struggle academically with things he previously found easy. All the outward signs (pale/greenish with dark circles under the eyes etc) are that he's physically unwell and that's the line the GP has taken in the first instance.
I can't help wondering though, whether this is all anxiety related, from him trying hard to cope and fit in and if I've been doing him an injustice in not getting it looked into sooner. He is just about at the age when ds1 started to struggle and things really went belly up when he moved into Juniors in year 3.
His teacher, who incidentally was ds1's teacher in year 2 as well, says she has no concerns about him having any AS traits - but then she failed to pick up on ds1's problems despite him being a lot worse.
I'm not much help really, as I'm in pretty much the same position as you in not knowing what to do for best.
As it stands, we saw the GP yesterday and they've asked us to see how he is after resting over the holidays, then bring him back and if he's not better, they will refer him to a paed for further medical investigations. At the first appoinment, they did mention the possibility of referring him to CAMHS for anxiety if the bloods came back normal (which they did, other than slightly low haemaglobin), but seem to have changed their minds about this now. 
In his case CAMHS wouldn't be much help anyway, as they don't do the ASD dx in our PCT, we have a specialist multi-disciplinary team who do that.
I was thinking about asking ds1's Ed Psych to do an observation of ds2 and see what she thinks, as she's someone whose judgement I really trust.
Is there anyone you would trust to do an informal observation to either set your mind at rest or help you decide to have her assessed? What's the SENCO like, would they be any use in the first instance perhaps?