People usually make the (low protien) rat block available at all times (the 2 recommended are oxbow and Harlan Teklad 2014)
and then feed a variety of vegs once or twice a day, and give occasional healthy treats like berries, a piece of fruit, cooked whole grains, organic pumpkin seeds, etc
btw rats need at least 2 sources of water (that does not contain chlorine or fluoride) in their cage - at least 1 needs to be a water dish that can not be upset (plus the tongues of young rats are not strong enough to move the ball bearing in water bottles although it may look like they are drinking)
There is a forbidden foods list
https://www.ratshackforum.com/threads/food-dos-and-donts.22782/#post-343892
and as you likely are aware, rats do not do well living alone - they can become withdrawn, depressed, stop eating, become fearful etc so getting a friend for him asap is essential for his health and well being
….. plus when introing rats. if all rats are under 12 weeks old then they can just be put together
But if any are over 12 weeks old then you need follow gradual intro steps so that no one is harmed - therefore it is much faster (immediate vs weeks of hard work) to get him one or more rattie friends asap because once he is 12 weeks old you will need to do gradual intros regardless of the age of his new friend(s)
for intros see
https://www.ratshackforum.com/threa...ps-used-when-putting-new-rats-together.32368/ and joinrats.com has a lot of good info
Edit: when bringing in new rats that have not been quarantined, it is a good idea to quarantine the so that if they are ill, your current rats do not catch it
http://ratguide.com/health/basics/quarantine.php