Threat or not, this isn't something I would take lightly...and whatever you do, do not reply to it! Especially since you have a kid to worry about. You should easily get a restraining order with this. Outta curiousity, were you able to get the IP off it?

I knew Dennis could comment on this better than I!  Good Job.... Stop this mayhem!

     This is a threat. Whether it is intimidation or real, it doesn't matter. Whether Donavan knows or not doesn't matter. 
     You have a responsibility to protect yourself and your child. You can't do that taking care of another adult that won't take responsiblity for himself. Nor can you live in fear because a bully wants to intimdate you. If he was that concerned about his brother - he should take responsibility for him. 
     Contacting the authorities would be a very smart thing to do.  Just explain the situation and ask them what can be done, that you just want to be left alone to live in peace. They have the access and resources to insure that this type of thing is stopped without fuss and mayhem.
Take care,
Dennis

Make sure you save the message.  I'm not an expert, but maybe it's just a scare tactic, but you don't want to leave yourself open either. Anyway-- could you just ignore this and this issue possibly go away?  This is a threat, so it can be reported.  Hmmm, let's see what others say.  You can't let others hold you hostage like this I feel.