Welcome to the forum
I have a rat who has always had problems with weight fluctuations. He is on a diet of veggies, fruit, corn flakes, plain Cheerios, and raw pasta along with low protein wet puppy food. Do you think this diet will help him gain weight? You can feel his loose skin and spine but he eats like crazy!! So a little extra weight shouldn’t hurt right?
No I do not think that diet will help him to be a healthy weight.
That is also a very unhealthy diet for rats. It will not provide the nutrition that rats require. Dog food is very unhealthy for rats as well.
Rats are fed a good quality block (Oxbow, Harlan 2014, or Science Select), available at all times
plus a variety of daily vegs, and occasional healthy treats such as berries, a piece of fruit, cooked whole grains, a few organic seeds such as organic pumpkin seeds, organic kumut puffs (cereal that is non-GMO and has no additives), a few cooked (dry) beans, a few cooked chickpeas, etc
Rat blocks provide the nutrition that rats need. They can be ordered from various stores and online stores, and purchased at most pet stores. Oxbow and Harlan 2014 can often be ordered in bulk sizes - people repackage into freezer bags as it is good for 9 months to a year frozen, but will go stale 3 months after a bag has been opened. Since a rat will eat approx. 1 Ib of blocks a month, a large bag (15 to 22 Ibs) will last a long time if you only have a few ratties.
You can find the ingredients listed on the internet for each the 3 blocks/brand names I mentioned
Please see the Reference Thread for good info on diet, nutrition, the forbidden foods list and other good information
https://www.ratshackforum.com/threads/reference-thread-read-only.35894/
Btw, if he is a lone rat he needs to live with at least one other same sex rattie, and although you are isolated, hopefully you have access to a good vet with the knowledge, and experience to treat rats and the necessary medications, etc as access to vet care is essential when owning rats