If your son spend too much ( >$50/month) for the phone, then he can talk to you firmly.
And why you let him pay your phone bill? Let him monitor your social life?
Always put your son first if your son is younger than 16 years old. You can change BF easy, not son.
I know a girlfriend who dated BF who only pay attention for her, not her daughter, not work out. She merried now. When her hu*****and (BF then) dated her, go out, buy small gift, he always buy two for each...
If someone dates a person with kids, this person has to have high EQ to make things work out.