Oh, I really feel for you.
With DS (my first), I was SUCH a complete newbie, I worried endlessly about everything!! (well, who am I kidding, I still do!!)
Poor old DD is so much more robust because, well, she just HAS to be!!
I'm no expert, but my two kids never had a 'neck' check (and they both had their 6 week checks with Paeds because they were born in the Portland). 6 week olds have floppy necks.
Just keep the Paed's appointment (when is it? You don't have a massive wait do you?) and see what's what.
Personally (And I appreciate I have no idea of your personal circumstance) if it was me, I'd take him to a Paed in the Portland. I end up taking my two back there because I rally don't get on with my GP (whole OTHER story!!)
Seriously, 99/100, this is NOTHING.
Just remember that you worry because you adore your little boy and what he needs more than anything else right now is your love.
HTH, HM x