My thoughts exactly chicken , a debt for his digs is unlikely to be
a) as little as £100 and
b) as round an amount
It is, however, a perfect amount for a fun weekend!
I'd be suspicious about his reasons for the handout and the likelihood of him ever paying it off.
If you and your DP share finances then it IS absolutely your business how he uses significant amounts of your money. And to the poster who referred to you 'only working 3 days' while also studying and presumably doing the lions share of childcare around all of that, here, 
It is hard not to compare parenting styles with someone you share so much else with. When you are constantly making the tough unpopular choices because that is the way to teach your DCs good long term habits, to have a partner who chooses the easy path, which does nobody any favours long term, is frustrating.
If it's any consolation, you can be proud that your DCs will grow up with a responsible attitude to money and work. I don't see why making that comparison with someone who can't hold down a job or a college course because he spends his day as getting wasted is such a bad thing. We all have our shit to deal with, nothing makes this boy any more of a special little flower than anyone else. Your own DCs have also had upheaval in their lives, but they don't use it as an excuse to opt out and get wasted.