This link has plenty of info, although you'll prob be well aware of most of it anyway:
http://www.nhs.uk/Conditions/Cellulitis/Pages/Introduction.aspx
I don't know why, but it seems that if you've had cellulitis before, then you're more likely to get it again and I've absolutely no idea why he's getting it in both legs.
I had it approx 20 years ago in my lower leg. I ended up in hospital for over a week on a drip. Anyway, it cleared up and thankfully I haven't had it since.
From what I remember it was quite painful.
I hope he makes a speedy recovery - he has my sympathy, as I kinda know what he's going through .....