He's never shown any interest before but suddenly he want to borrow my bike and find out what I see in it. I'm confused. It's always been my outlet. He's got whatever he does on the internet and his cars. What should I say/do? I'm secretly hoping he hates it.

point saddle up, soft tyres, remove the granny ring, really sharp front brake – there are so many options

single speed rigid and lycra

Why cock-block him? He might really like it and you'd have another shared interest in common. He may be looking for a way to spend more time with you and less time on his cars and porn addiction, sorry i mean "internet interests"!
I think you ought to encourage him!

His mates probably keep telling him how fit his girlfriend is……..

Realistically he may enjoy it, but he has other long-standing passions this probably won't take over and actually it's really nice to get out together. Although of course you will get annoyed when he rides straight down something you took 3 attempts to do (bloody boys)!!!!!

There's always rides to the pub without arguing over who's gonna drive. 8)

Perhaps he's jealous, of you meeting fit young men whilst out on your own, and wants to monitor you.

Never assume theres a nice explanation, when something more sinister lurks. Its basic human nature.

'cock-block' an unusual use of a 'pick up artist's' term in this situation.

Have you been reading Neil Straus?

Be carefull with his fragile ego, we are delicate really!