I told DH about the content of this thread, it brought back memories.
We had this in our family too, saffron, around 14-15, up until DS turned 17. Small scale and gradual. Basically the alpha male was suddenly realising that the next in line was threatening his position. Mealtimes were the flashpoint. Just as you think isn't this nice, all 4 of us sitting together, pow. I don't mean it ever descended to physical btw. Sometimes it was a hair's breadth from joshing around to anger. But I could see when they pushed each other's buttons. Nobody likes a smart alec but DS couldn't always help himself. At some point DH got riled enough to stop making allowances for his youth.
Now in your case it doesn't sound as if DS is doing anything wrong.
Unless DS brings home report cards and gets detention weekly, he obviously behaves himself at school.
Week days they'll have school and work respectively, what happens at weekends, do they spend time together?
What was DH's relationship like with your FIL?
I doubt DH hears himself, how he comes across. How useful and enlightening if you could somehow record such a scene.
I feel sorry for DD too, if DH doesn't realise this will impact on her he is kidding himself.
The strange thing is, if my DS hadn't ever challenged DH and "clash antlers" and develop his own personality DH would probably have felt he had failed as a father.
DH finally accepted DS is his own person, thankfully before DS left home to study. He says now that he has to rein himself in on occasion but on the whole they get along now because they respect and love each other.
Is there some sort of Family Counselling service available?
I have now read FIL was a lovely man. DH shouldn't feel threatened by the next generation. I don't think you'd have married a bully, so I don't see why you should put up with one now.