Do they have access to furniture or will they be looking for a furnished place?
Additional expenses -- totals listed here were split equally among roommates: Most apartments here require renter's insurance with a minimum of $100,000 for liability & include personal property coverage of $20,000; DS has been paying $15/mo. Usually the renter contracts independently with an electric company; DS pays $55/mo. average. His apartments charged for water & trash, about $25/mo. He uses internet only, wired & wifi, about $50/mo including the modem. No landline; he uses a cellphone.
Generally the best idea is to split the rent itself according to the square footage of the personal space -- if one roommate has a bigger bedroom (or the only bedroom), then he pays the larger prorated amount. It's a bad idea to pay based on income, IMO; if one of them loses or quits his job, does he then get to not pay rent? Sleeping in a common space creates its own set of problems, especially if that person isn't very tidy.
Does your son know how to cook? This is a huge money saver, not to mention better nutrition. The only small appliance DS took to his first apartment was an electric Instant Pot, which has a 10" diameter footprint & functions as a pressure cooker, slow cooker, rice cooker, egg cooker, steamer, saute pot, & food warmer. Set it & forget it until the food is ready, & cooking with pressure saves energy & tastes better than other methods. DS' roommates used it too. One summer internship housing did not include an oven, but the Instant Pot was a good substitute & not every place has a microwave.