My DS1 is 18, a straight A sixth former (hopefully) because he is self motivated and hard working. He spends EOW Thurs after school to Sun evening with exH and his new wife with his brothers, 16 and 13 and new wife's DS, 12.
Today he missed school for the first time in years, he's hardly ever off sick and we don't do term time holidays. He arranged to go up to London with a friend to see a band. This is very new territory for him, he is pretty much a stay at home geeky lad except for Explorer Scout events and has never been to a festival etc.
He told me about this trip before Christmas, but as it was on a weekend he's at his dad's I said he'd have to discuss it with him. I reminded him this week to make sure he or his dad informed school he'd be away. Otherwise, I was quite happy for him to go. I didn't think one day was a big deal and thought the experience would be good for his social development. The friend he's going with us also very hard working and they'd organised a travelodge near the venue to stay in tonight. (Very rock and roll! 
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I've received an email from his dad who is very unhappy that he has skipped school, and equally unhappy that I haven't informed him about it and appear to approve. Apparently DS1 only told him last week about the trip and had said he was leaving after school. ExH found out this morning it was a whole day trip and that I already knew about it.
Trouble is we parent differently. I trust my DS1 pretty much to do the right thing. He is a well behaved lad apart from normal teenage sarcasm and is after all, an adult. ExH has become a very strict parent since he left us, maybe under the influence of his new wife and her much younger DS. Until recently DS1 has a 10 o'clock bedtime at his dad's and isn't allowed to have friends back unless exH is there etc.
I first met exH when we were 18 and we both had much more freedom than DS1 does. I had a Mini age 18 and drove groups of friends all over the place. We went to the pub every Friday and Saturday evening, both lost our virginity at 17 (not to each other!) and yet we both managed to go to university in the 1980s and get good jobs. In fact, I was much more of a goody goody than exH, who used to drink to excess regularly. I think he's forgotten how old DS1 is and still sees him as the 13 yo he was when he left.
So, AIBU not to discourage DS1 to let his hair down for one day? Is exH BU to expect me to inform him of DS1's plans, when DS1 as an adult, is perfectly capable of informing him, himself? Or do we both just have to accept that we have different parenting styles and standards?