I recently had a freak out related to a new bump too, I so know where you are right now. The most important thing people said that helped talk me down was that osteo is extremely painful the most stoic dog would still let you know s/he was in pain.
I don't know enough about an IV site developing scar tissue, but I will say that we determined the bump I found was just that, scar tissue.
If he's comfortable or doesn't freak out when you touch the site the possibility of just scar tissue is high.