Don't let him be your sole source of information about BDSM. Do some research yourself - find a local munch [meet up of people into/exploring BDSM in a pub or similar everyday venue] and meet other people you can talk to about it. Usually because of where munches take place you won't see people doing BDSM at them and usually people will be dressed in ordinary clothes Ones where it's OK to do BDSM are advertised as such, so you are unlikely to see people doing kink things you don't want to watch unexpectedly.
Often the less than savoury characters are well known to people who do kink; which is why some abusive people (nearly always men) will pick as potential victims people who are new to BDSM and who don't have access to other people's knowledge about them. If you go to a munch and meet other people who are into kink you're likely to get yourself into a position within a few months of knowing who is who and having gossip passed on to you.
Online, there's FetLife, [obv not safe for work] and other sites giving information re kink. There are also books - I think my starting recommendations would be The Topping Book and the Bottoming Book by Janet Hardy and Dossie Easton, but there are lots of other good books as well as books that aren't very useful.
It's not "normal" in relationships where people do BDSM to try to stop partners meeting other kinksters at munches or other events. It's not normal at munches to be doing BDSM play of sorts where someone "isn't allowed" to talk to other people. Trying to isolate you (or keep you isolated) from the "scene" / "community" or whatever other term you use is a red flag.
medium.com/@hermessolenzol/how-to-recognize-abuse-in-bdsm-relationships-77816e8ee2ca is good.
www.vice.com/en_us/article/panjbz/how-to-tell-if-your-bdsm-relationship-is-abusive is possibly useful.
polyamory-metoo.com/ is also worth looking at, even if you're monogamous, around the ways in which women were emotionally (and otherwise) abused by someone who was seen by others as a pillar of the community. This is the sort of grooming someone who is clever might try to do to you.
Another couple of links: www.nytimes.com/2018/05/23/style/bdsm-kink-consent.html
www.thehotline.org/2015/02/13/everyone-deserves-a-safe-relationship/
You can decide you're not interested in any BDSM type stuff or only into bits of it or that it's something you might want to explore sometime, but you're busy doing other things so it's not a priority. BDSM is a very broad chuch of interests/activities.
Leaving all the kink stuff to one side, any partner telling you you're vulnerable should be looked at suspiciously. Someone you've been seeing for a limited period of time, making that sort of comment doesn't sound like someone you'll want to be involved with. He's maybe grooming you to abuse you, but even if he's not, it's pretty disrespectful to be judging you as "vulnerable" when he's someone you're seeing, not someone with a professional connection or someone you've been close to for years.
Sensible people who do BDSM don't want to do it with people who they perceive as vulnerable. There are, in England, [generally theoretical] legal risks to some sorts of BDSM play - nobody wants to find themselves arrested for play they thought was consensual. Why put yourself in that position, when there are lots of people who are interested in / willing to explore BDSM?