There are plenty of rodent foods on the market. They contain the correct nutritional requirements for the rat and should be available for him all the time. You should supplement the rodent food with fresh fruits, vegetables and greens. Make sure you wash all fresh fruits and vegetables. You can also give your rat whole grain breads.
Cat food is too high in fat and protein for a rat.
Make sure he has plenty of fresh water and change it daily. The first couple days leave the rat in his cage, but spend small amounts of time talking to him. After he adjusts to his new home start offering him treats with your hand. Treats like banana or cheerios work well. It may take a little time before he'll take the food from you.