I pay £8 a month for my mobile services and it's all unlimited.
It's never a good idea to get on benefits especially at such a young age.
It trains the brain in an unbeneficial way, not just for him, his younger brother but the society as a whole. The absence of his biological father will not help either. It teaches lack of responsibility.
When a youngster gets their first job, their priority should be gaining experience and saving, not access to on-demand sex. Self-control and discipline are virtues that will serve him throughout his life.
A recent graduate/leaver, should stay home, build their career and treasure/war chest, then move when they have that savings, so that even when he/she looses their job they can go for a temp role to cover basic expenses and keep the habit of work routine going. Junior roles, carer, supermarket, shop, admin, events, retrain explore all.
Then hopefully progress a current relationship to marriage or build one with someone of similar minds.
He's your son and family comes first. It's either you take care of him or the tax payers do. He should move in with his brother. Learn/help to care for him, as he would his own child, build a stronger bond with him and get any legitimate job he can find. Even this experience will teach him, strengthen him and prepare him for his own family. It's better he experiences the fleeting nature of money now while he has no children or wife but if he fails to experience the discomfort he wouldn't know how to make hay while the sun shines and plan for the rainy days. PS: the economy is set to get much worse. He can aspire to get benefits and Council housing or learn from this and aim higher.
This is an opportunity for him to build not just his own resilience and future, but also build stronger bonds with his step father and half brother and prepare for his future.