Couple of things. First (as an ex fifth grade teacher), he is a little young which could account for some of the lack of friends. You might ask his teacher how he fits in age wise with the other boys.
And yes, the new baby, you, perhaps even the thought of middle school is going to cause him to seek attention so that could also be part of it.
However, when you get two teachers telling you (or suggesting) that he may have ADHD - this should not be ignored. If these are experienced teachers, they have seen a lot of kids and are comparing his actions to the hundreds of other children they have seen - and he stands out.
But more importantly, the fact that dad apparently choose to ignore this advice because - "dad was opposed to him taking medication." shows a complete lack of knowledge about ADHD and what it can do to a child.
First, no one can make the child take meds. Second - and more importantly - there is a lot that can be done without meds. And even more important then that is the long term consequences of the child trying to go through school (possibly with ADHD) and not knowing why he has no friends, is flunking classes, getting into trouble, etc. Those kids can have very serious consequences later in life - self medicating with drugs for one.
Finally, if he does have ADHD, the way you handle discipline is also very different. Many of the normal methods won't work. I can give you lots of resources on that - or feel free to post over on the
http://www.medhelp.org/forums/ADD---ADHD/show/175 where I am also the CL.
I would suggest that you check out this link - http://www.help4adhd.org/en/about/what/WWK1 and read up on ADHD. Make sure that dad also checks it out. I am here if you need more info - or over on the other site for that matter. Best wishes.