We just added a set of ACD's that are 10 weeks old. This is our second go with ACD's, our first are 13 y/o. We did a great job training and socializing them back then. My question is, one of the new puppies seems to act aggressive when my 10 y/o picks him up...what could this be? I didn't have this problem the first time around. And we just started having the kids work with their training, too. Thanks.
Aggressive how? He's obviously not comfortable with your son picking him up; simply do not allow your son to do so...
A lot of dogs hate being picked up. If he only reacts when your son lifts him, it may be that he holds him awkwardly, or holds him too tight, or he doesn't feel safe in the hands on one so small. I agree with BlueJay. Why push the issue. Just ask your son not to pick him up, but instead to play (gently) with him so that they build a bond that way