Hi Rona,
Mastitis and blocked ducts suck! It happened a lot to me with my first child. What seemed to stop the problem was switching to a less constricting bra. I had been using one that was kind of like a sports bra, but when I would take the flap down to feed, it was putting pressure on breast tissue and causing blocked ducts... which can lead to mastitis. So maybe try a different bra.
My lactation consultant had also recommended lecithin, which is a nutritional supplement. I can't say if I think that helped because the problem really seemed to clear for me just by switching bras.
If I were you, I would just let your baby feed as much as he wants on one side, and then switch and let him have whatever he wants from the other. A lot of books and stuff write about feeding a certain number of minutes on each side, but your baby knows how much he needs to nurse and will take what he needs - hence the supply/demand thing. I also had an overproduction problem and was feeding a certain number of minutes on each side, which led to my breasts staying rather full ALL the time... and worsening the duct problem.
Most hospitals have lactation consultants, so if you're still having trouble, by all means give them a call.
Good luck and hang in there.