Hi there, glad Mortimer is doing well! I have a single male mouse as well that I adopted from a shelter. From what I understand, male mice will not likely adapt to another mouse after being separated at a young age. Even neutering an adult may not help at all. I've learned to keep mine single, even though adding a buddy would be nice. I think if it can happen, a neutered male and a female are the best match? I could be wrong. I've been advised not to pair up mine though.
For the food, I use Reggie Rat, bought at Petco. The recommendation is appx 12-15% protein, 5% fat, and 3-5% fiber. Another food that fits the profile is Essentials Adult Rat Food I think found at Petco or Petsmart?
For a cage, you have several options ranging in price. The cheapest is a bin cage. Side ventilation would be ideal, but you can also get a tank, such as a 20 gallon long. A much more expensive option is a mouse cage called the Marchioro Kevin. I got mine at Thatpetplace but last I saw it was out of stock. It's huge and my young Syrian is currently using it. Lastly, the cage my mouse is in, is a meshed Guinea Pig cage. He really loves climbing all over it! Meshing is a huge pain, but if you're up for it, it can be a great cage. Here is a pic of mine (I've since changed the zip ties to clear lol):
