My new rat sneezes constantly (myco)?

I got my new rat at Pet Supplies Plus feeder section. I got a small rat that I ume is about 2 months old (not full grown at all). I have had him for about 10 days. He has been sneezing frequently since I got him. He otherwise looks/behaves as a healthy rat should. Only recently, however, have his nostrils flared up and looked a bit red and gunky. I have read that this is myco, and it is highly treatable. But so many websites give such different information. I don't want to take him to a vet, I'd rather try giving him medications first. But I have no clue what dosage to give, as he is very small. Any advice is appreciated.