As you’ve originally said there’s the ‘good side’ and as pointed out by other posters there is the kinky side, but more relevant is as highlighted by Lang that predators will exploit these areas
There are furry & cosplay clubs / societies around the country, I go to a few comicons and a particular cosplay society is prevalent at them. There are quite a few furries, I don’t know if they are in that society or have their own groups - but all tend to be friends and associate with each other
That society (appears to be) young cosplayers running their own society for their own benefit, and the event organisers are very hot on what is and isn’t acceptable at their events
There are a few online groups that flag up warnings and particularly flag up events/organisers that are more ‘suspicious’
As you say, you only have limited influence on an 18 year old, but keep engaged and aware where possible
Is he a member of a group, and can you see the range of people associated with it?