I had this in college (don't ask, I still don't kow how I ended up with it at 21) but mine started with strep throat. Could he be suffering from swollen glands or strep? I would imagine that would make him pretty irritable. I also ran a low grade fever the whole time, but I'm sure even that would affect a little one much more than it did me. I know I also had very little appetite that whole week. If I remember correctly, the rash is the most persistent part of roseola, and the other symptoms pretty much last a day or so before moving on.
I hope he starts feeling better soon! I know my rash was so red and noticeable, some people thought I had sunburn, and others asked if I should be sitting in a bathtub of ice to cool off. I do remember that it all started on a Monday was done by the following Sunday, so hopefully it won't last too much longer for him!