Hello XandaPanda and welcome.
Please don't feel bad, I have a ds (now 10) who has Asperger's and I didn't realise there was anything different about him until he started school either. He is also my eldest dc, so I had noone to compare him to really. He didn't get his diagnosis until he was 8, because his teachers didn't notice even once we had started to suspect and raised questions several times with the school.
AS can present in quite subtle ways when children are younger, which is why a lot of professionals try to persuade parents to go for a 'watch and wait' period rather than going for assessments straight away. This of course is how children like my ds get overlooked and imo and I think - I hope - this sort of delay is now less common.
It's good that it has been picked up so early, as this gives your ds the best chance of being properly assessed, diagnosed if appropriate and supported through school.
Don't panic and assume that he must have Asperger's just on the say so of one GP though, there can be a myriad of different reasons for similar behaviours and the only way to get to the bottom of thing is via a thorough assessment.
This bit is really hard. The waiting is awful and in the meantime your brain is going 200 miles an hour constantly churning all the stuff you've read over and repeating all the 'what if's'.
Do keep posting on here. There are lots of us on here who have children with ASD and there's usually someone around to listen and answer questions or just offer a hug and a
if that's what's needed.