My little cutey looks similar but the vet told me that it's not always that rats get fat because of too much or false food. They can also get adiposity by genetic reasons or genetic reasons may be supporting for adiposity.
The vet also told me that it's important for adiposital rats to watch careful for their weight because loosing too much of it at higher age can cause sudden organic failure exspecially for liver and kidney so as long as he is fit cuddle him but just be careful when he gets older
By the way, he is a really super cute guy. I love his cheeky expression and his back feet looking like he starts paddeling soon like in a tabby dance oder something