This thread disturbs me. I understand being concerned about the girlfriend affecting the son's school performance, but is that worth involving social services over? And do you not think that the more you try to come between them, the more forbidden and passionate they will feel it is?
And as for everything else - unfavourably comparing your son to his brother, complaining that he doesn't respect you (news flash - teenagers all hate their parents, it's not personal), complaining you're going to kill him etc...
You sound like how my father described me when I was 15. He would make out that I was the most horrendous, insolent, off the rails kid in the world instead of a virginal straight A hard worker who was actually just 15 and therefore had the odd mouthy teenage strop. Like you, he couldn't accept that teenage strops are not personal, that all teenagers hate their parents by default and as the adult, he really had to not rise to the damn thing.
Instead he took them as great personal affronts to his paternal authority, and would scream, swear, threaten to kill me, try to chuck me out of the house, and of course physically assault me (slaps, kicks, punches, thrown objects), all the while blaming me because I was, like your son, a nightmare, a disaster, and, worst of all, DISRESPECTFUL (what idiot expects respect from a teenager, seriously).
I'm really siding with your son on this one. I'm sure he's a royal pain in the arse, welcome to having a 15 year old. Do you remember being 15 and how shitty and confusing it was?
But you and your husband need to be the bloody grown ups and not escalate the situation by seeing it as a dreadful affront to you personally as opposed to just a teenager being a bloody teenager.