I know it's hard not to worry when you've got a little 'un but truly, don't let what she said get to you. If you think Alex isn't growing the way he should, get him checked out, but otherwise....
James is ten months today (happy ten months James!) and I accidentally stuck him in a 3-6 months top yesterday, and it fitted! He's wee, he's full of energy and like Alex he was 7lbs 10 at birth. I am kind of aware that he eats less than other babies, but I can't forcefeed him (and neither would I want to!). And as for the walking, we're not there yet but babies take their time - and if he's got big sis to get him stuff there's no real need to walk!