Advice Needed on Consolidating Web...

Not much experience with this... but I think you could get the most juice flowing to the new site by 301 redirecting the old 5 sites to the new.  This would consolidate all the links to the new site... which is where they belong.

understanding-the-301-redirect/

I would consider hiring a SEO who has done this type of consolidation successfully in the past.  That is a much better idea than your boss had

Lucky you!  You're in the catbird seat -

 I've been through this before, and it's really great to suddenly have 5 related websites that can all link to the new one however you want, (anchor and url's).  Even a little better if they're all on scattered hosting accounts!

My advice is to build the new site, then if desired, you can rebrand the old sites to look somewhat like the new one. Then turn each old site into a sort of "announcement" that the companies have changed / merged / etc. 

You can keep all the old URL's (especially ones that have PR) , and the content too, (obviously edit what's no longer accurate), and then add text on any / all pages you like with well anchored and relevant deep links to the new site.

Assuming the old sites are indexed, and have even a little link juice , you really be in a better position to kick start the new business... unless of course there were 10 old sites!

In my opinion, don't miss out on this huge the opportunity you've been given by just 301'ing the old domains. That's certainly better than letting them die, but not nearly as  good as suddenly controlling a small network of domains for your new launch.

After a few months, you can kill them off one by one with 301's  if desired, but personally, I like keeping them ;)