It sounds like mites or lice. But I don't have a picture so it's hard to tell. Have you ever considered making your own food? I stay clear of bagged rat food due to them using certain chemicles, dried corn etc. Here is my feeding article:

http://www.rompingrattiesrattery.com/feeding.html

This may help you. Could you take him to the vet?

Looks like that food is better than most on the market, so I wouldn't be too worried.