Please help!

[Llaison]

You mention that he is not fully grown - how old is he?
Even with that many birds, it doesn't mean that the 3 fully grown roosters didn't attack and hurt him badly.

He was hatched this july

 

[Llaison]

I’d like to find out more about the vaccine. How would I go about vaccinations in my flock? (Do ages matter?, if it’s going around can I still vaccinate?)

 

[Wyorp Rock]

I’d like to find out more about the vaccine. How would I go about vaccinations in my flock? (Do ages matter?, if it’s going around can I still vaccinate?)

Are you referring to the Marek's vaccine?
It would be administered within the first 24hrs of hatch.

Here's some information about Marek's for you
http://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq