Its around this age (6-7 months) that they realise they are seperate entities isnt it? That they are not a part of there mummy, they are there own person?
My DD is nearly 8 months and will tend to cry or whinge when i put her down to play. If i walk past the room she is happily playing in, she will let out a cry or a whinge.....i know its only coz she's seen me, and wants to be picked up.
I totally, totally understand that you cannot pick DS up every time he cries. I dont know about you, but 'NO' just doesnt seem to sink in at this age, not with my DD. Have you thought of introducing a teddy, or a wrap or something as his security object - something he can give cuddles too and form some kind of bond with - help him understand he can have relationships/fun with something other than mummy??