Many, many years ago a close friend's dad was fairly senior in the Merchant Navy, and spent months away from home. There were three children in the family, the eldest a son, then two daughters, the elder of whom was my friend (though the son was also my first boyfriend ). As a teenager I noticed a great deal of tension between father and son when dad came home, because dad was accustomed to giving orders and being instantly obeyed when he was at work, and of course the rest of the family functioned quite differently. The son was effectively "man of the house" while his dad was away, and he didn't take kindly to the instant demotion when dad came home.
I'm now seeing the same thing with the teenage pal of my DS2 (they're both 18) and his dad, with the added complication that he's actually step-dad. There are two (considerably) younger children who are dad's natural children, so it's also possible that dad favours them over his step-son.
Is this common, and is there anything I can say to the teenager to help smooth things over at home, or should I just keep out of it? The teenager has just become my lodger and the story he's told me about why he left home doesn't quite tally with what his dad said to me last night.