If you've exhausted all the other possibilities, meaning he'snot sick or refluxy or what have you, then he's probably just worked out that you are NOT a part of him ... that you are a seperate entity, and therefor you can go away ... and come back
I know it is not much help but sometimes they just do need their mumma a bit more than usual.... for no real reason other than he has acquired a new skill he wants to show off or because he is feeling a little overwhelmed with all the rapid development and is looking for comfort.
Yep, totally agree with Diva and Carley. Oh, and Dee too about the Wonder Week suggestion. Not all baby's are right on the wonder weeks schedule but, as it says in the book, they'll be roughly close to the wonder week age and the changes are usually pretty spot-on.

At this age, he doesn't know how to manipulate you. All he knows is that he has needs (food, warmth, security) and that mum seems to be pretty great at meeting all of them

Sounds like he's just a little overwhelmed. Remember, pretty much everything is new for him and even though we all get excited when they reach a new 'milestone', it's a whole new world for them. The way I think of it is, imagine if you woke up today and you could fly. No-one has mentioned flying, the thought had never crossed your mind before, you haven't seen anyone else do it... I think you'd be a little freaked out and overwhelmed and in need of some reassurance! Pretty much every week he realises he can 'fly'

You're doing a really great job, to you xo