I just went to webmd to look up roseola (which I think that looks like to me).
this link has a slide show, and this definitely doesn't look like the hives they have pictured:
http://children.webmd.com/slideshow-...-skin-problems
My kids both had roseola at around 6 months. They had fevers for a few days, then once the fever went away, they broke out ALL OVER with a rash like the one in your pictures (their rashes looked more like yours than in the webmd pic). They acted like they felt fine the entire time they had the horrible rash, too.
I would say not hives...does he seem to be itchy? I know, hard to tell with an infant! but if he's not showing visible discomfort/frustration, I'd vote that it is NOT hives.
HTH!