Others have already said this, but I just thought I'd add another voice saying it's fine to carry around your other child! There's no established limit on how much to lift, but "avoid lifting heavy objects" is a pretty common statement ... I've seen "heavy objects" described as anything over 10 kg (which is 22 lbs, much less than your son weighs :)
This early on, it's not much of a concern, but one the baby and the belly start to get a little bigger, it's generally a good idea to avoid lifting very heavy things. As long as you don't have a high-risk pregnancy, this basically means don't do heavy weight-lifting exercises, don't move furniture, etc.
Here's an article from Baby Center where an OB actually says moving furniture in the 1st trimester is OK, but to stop in the second http://www.babycenter.com/406_is-it-safe-to-move-furnitur....
I hope that helps to put your mind at ease!