Was there a lump there previously? I am wondering if he had an abscess that burst, such as a preputial gland abscess
Here are some links re caring for an abscess
If it does not improve or get worse you will want to take him in to see a good vet with the knowledge and experience to treat rats, asap as he may need antibiotics or surgery
Given the location, it could become very serious
http://ratguide.com/health/integumentary_skin/abscess.php
http://ratguide.com/health/figures/abscess_figure_5d.php
I hope Oz is soon feeling much better