As long as he isn't in danger at his mother's house, there isn't anything you can do. Having said that, I suggest you (plural) are brutally honest about what will happen if the gf gets pregnant, to wit, he will be forcibly emancipated and turned out of your home, he will have to get a job to support his new family, school will be over and along with that, any hopes he had for his future. Put the fear of god into him. 

Also, make sure your DH has been perfectly clear about sex and contraception, even if it means making condoms freely available. 

To avoid pregnancy, it might be worth contacting the girl's parents to see if they care enough to put her on BC.  Or maybe they could take the car away from her.

Start calling SS "pops" and definitely have a discussion.

It's exhausting to be powerless and watching these kids take dangerous paths because the other parent won't parent!!

I would start by having the sex talk with him in real terms, no sugar coating or dancing around it. Buy him condoms. Tell him to not trust that she is on BC. Show him how much kids cost! Take him to the store for a lesson on diapers, formula, clothes, toys and everything else babies need. Explain that he will be expected to work and pay child support for the next 18-21 years. That college is over for him, he won't be able to afford it nor have the time because the baby will need things. No more weekends with friends. Nothing else but taking care of a baby. 

There are exceptions with people being teen parents and being able to have a successful life, but those are exceptions- not the rule. 

We went through this with OSD. BM just threw caution to the wind and let her go out and about with her first boyfriend with no supervision. She of course swore they weren't having sex, but YSD snitched on her. So we had really serious conversations about exactly what would happen if she got pregnant. It would effect us more than BM because BM has no money, so we offered to pay for BC that wasn't a pill - something more or less foolproof. She opted for an IUD. I still remind her all the time to use condoms because some stds are lifelong sentences. 

If your SS's girlfriend is also a minor, I personally would encourage DH to reach out to her parents. Just tell them that he isn't comfortable with them being alone, and he would appreciate it if they respected his wishes. He obviously can't make them do anything, but it can't hurt for him to make it known that he's involved and disapproves. Best case scenario, they'll stop their daughter from going out alone with SS. Worst case, they just ignore DH. 

BM1 allowed SD21 then 12 yo to have a bf and spend weekends at his house. At 16yo, SD21 was pretty much moved in with her bf at his parents home and now at 21yo she supports him fully. Its miraculous that she never got pregnant. For SS20, she was his "wingwoman" when he was 14 years old and lied to one of her friends who was 28years old that he was 22yo. The 28 yo friend slept with her 14yo son with her help and she thought she was a "cool mom" for it. At 17yo, she allowed him to move in his gf who was 15yo and they had violent fights until the cops were called. BM1 has a history of sleeping with both of her sons friends who were barely legal, so thats that.

BM2 allowed them to date and have girlfriends as soon as 8 years old and she lets the girls come home and spend time in their rooms with door closed. SS16 spends entire weekends at gfs homes and has already threatened suicide twice over breakups with 2 other girls. SS14 has multiple gfs but nothing serious, they come over to his home and stay in his room with door closed.

You or husband cant do anything about this because the BMs will always prevail since they are CPs and their primary residence is with the children

My husband tried to control that stuff with all his children but he was gaslit as a reactionnary crazy person and the children think of him as someone who wants them to be "robots". SS20 has had 2 pregnancy scares and told my husband that wtv happens he will not be marrying no girl over some kid and BM1 told him its ok the girls can get on welfare so you dont have to worry about that. My husband asked him if they were good enough to sleep with unprotected, why are they not good enough to marry and have children with? His response is that his fathers thought process is ancient and no one thinks like that now. My husband told his kids that if they cant use protection while having sex, then they need to abstain, they called him crazy.

SS20 has had to go to the ER twice this year for STIs...

Best advice is for your husband to tell the BM that if a pregnancy or STD happen while the child is a minor, she will be fully responsible to support the costs and responsibilities associated with that.

Invite the girl and her parents over and have a frank discussion with all of them, that you do not condone SS14 dating at this point and that since the two are considering having sex or are having sex YOU will not be responsible should the girl end up pregnant, that responsibility will fall solely to SS14. 

I would also have a very honest discussion with SS14 what his life will look like as a teen father.  Which would entail getting his GED so he can begin working full time to support the BABY, not baby mama, and a custody agreement will be worked out with the court system.

SS is 16 and has his first real girlfriend. He is basically unsupervised by BM and according to him, his girlfriend's parents don't care if they have sex at her house. Every time SS mentions the girlfriend, DH tells him that he needs to use a condom every.single.time. He also told SS that he should suggest that the girlfriend get an IUD. DH figured that being as direct as possible would be the best he could do as the EOWE dad to ensure SS was aware of the need to prevent pregnancy.