This is the type of issue that all three of my kids are afraid of me on. NOT because I have abused them or been awful in any way to them, but because they KNOW that I would report it to the police and CPS.
Nowadays, if an 18 yo and a 14yo are dating and living in the same house they are not skipping sex. That is just how it is now. Even if the mother tries to make them sleep in different rooms, the teenage sex drive is just way stronger than mom's rules.
I married a guy who was over a decade older than I am. Still married to him, in fact. After we met we realized that he student taught in the building I went to jr high in - that same year. I didn't have him, he was there, lol. However, we did not meet until I was 21, we were not even acquainted before then. As adults the relationship was not a problem, it has been a really good one for over two decades now. But if I had been a teen? It is abuse.
I think you need to report this mom to child protective services, anonymously. Tell then that the daughter if 14 and has a live in boyfriend who is over 18. You also need to check the laws and push for a welfare check on the daughter. If they get pregnant he can walk away, but she will have to deal with those choices for the rest of her life. I would NOT leave your name at CPS or the police, not yet.
As for his things, let your son take the cheaper clothes, etc... but not the expensive stuff. or the ipod, etc... LEt him know that they are yours because you paid for them, and either use them yourself or sell them. Heck, pawn the electronics and let him have the pawn tickets to 'buy" them if he wants to spend his money on it.
BEFORE you do ANYTHING with his electronics, check them for sexual pics of the girl. If he has one, it can be a HUGE problem for him as it is considered kiddy porn. As the car is in your name, and you pay the insurance, etc... I would either keep it or consider selling it. If you have a younger child, maybe keep it for them. Your son knows you do not approve, and should not expect you to pay for a lifestyle taht you do not approve of and is against the law. Be sure to go through the car for evidence of a relationship.
If the police blow you off, hang up, stomp your feet in anger, call back and ask for sex crimes. Then make the report to them, NOT to the officer who first answers the phone.
Once he is arrested, or pulled in for questioning or whatever, I would let him know that this is what happens when you are involved with jailbait. He needs to know this can impact his ENTIRE future. And hers. his legally, hers emotionally and physically and likely legally. The mother can also be in a world of hurt. It is likely that for a first offense he will NOT get more than probation and an order to not see her. but the mother may get a LOT more.