Is this odd?

So my sister has a boyfriend, they've been dating for around 9 months and I've met him a few times and noticed that he's very quiet, but I've put it down to him being a little reserved and shy. Anyway, we all went away for a long weekend in the countryside and I've picked up on some things that I'm not sure if they're red flags or not: he sticks to her like a shadow, one instance she went to the bathroom and I went to get something from my room, when I came out I saw him waiting for her outside the bathroom - not to show or tell her anything, literally just to wait for her (we're in a cottage we rented and it's just us). Me and her were also having a private chat in the garden while the dog was outside, as I hadn't had much chance to have a one on one talk with her, and after a few minutes he comes outside too and just stands there. He doesn't input in any conversation or join in, despite efforts to include him numerous times. Plus, my sister told me he seems to spend all of his money but he hasn't told her where it goes, for example he just got paid last week and only has £60 left, he works almost full time, lives with family, doesn't pay rent only bills for his car. When my sister has asked where the money goes he replied with "I'll tell you one day". Should I be concerned? He's 23 next month and my sister just turned 20. Any help would be much appreciated!