Flirty Leo Male

So, I'm hoping the Cool Cats can offer a little insight here.

I'm getting chatted up/ strongly approached online via Facebook by a Leo boy...
He's Gem moon Sag rising....
Anyhow, the only reason I'm bringing this here is, I'm in a very good relationship which is posted all over Facebook ie we aren't 'that' kind of couple but you know, weekend brunches, family hang outs, friends etc are all mutual and shiz gets tagged with us together all the time so it's very obvious I'm not single.

I get the feel he is just super flirty, and having fun and that's cool and all, but the kicker is, Leo and I are like childhood mates. We haven't been in touch for ten years but we grew up same neighbourhood, school, high-school, same clique and classes all that jazz. We were never tight but you know how it is. Just kids I can hardly imagine him like that, we had popcorn sleepovers and after school bike rides and all that innocent stuff.
We've never looked twice at each other, never interacted once high school ended, live in different states the whole six degrees but never a real life thing but he is REALLY laying it on thick...
I just keep it light and laugh it off although I did say 'you are being way too flirty for your own good' I try to make it stay on track with convos but he keeps steering it back into more intimate territory.
Basically, out of respect for my other half I don't think this is appropriate or should continue but leo is a nice actually nice guy and he's single and probably flirts with every girl so thats totally his perogative, but how do I state a boundary or what have you without wounding his Leo pride.
I am flattered and would really like this to just disappear amicably.
Or am I dreaming? Is it the axe? Full stop.

Think it's the axe...he knows you have someone else but is trying it on anyway. You haven't led him on in any way have you?

Some guys need to be told point blank and even then they aren't hearing anything that they don't want to...married men chase and chase even after they are told to stop it!!

I would STRONGLY tell him NO and if he is persistent after that then Delete

We hate to be ignored. Ignore him or stay off FB for a while.

This guy sounds like he needs to get a clue, pronto. I know someone like that myself and he did the same thing. You need to distance yourself.

Here's what I did:

If the chatting is happening on FB, you can easily block him from chatting with you. Go to the chat bar, and click the little gear for options >go to advanced settings > then click "Turn on chat for all friends except..." and type in the Leo's name there, then he won't be able to see when you're on to chat with you while both of you will stay as FB friends. That might stave him off for a while.

Unless he blows up your inbox with messages while he thinks you're not around, idk. Then it might be the axe.

Thanks very much, guys. All good answers!
I understand the ignored thing, but I hate to come across as rude...
I'm just going to be more direct and say something.
He's a childhood friend, if he gets offended when I state I have a boyfriend and I don't think our chatting is appropriate than we obviously aren't suited to be friends at all. And that's just reality, unfortunately.
He's very charming and a lovely person I'm sure he will be graceful...
I haven't lead him on at all, it's completely out of left field. I'm not his type, I thought anyways.
I've known him nearly twenty years, you'd think he'd have made a move earlier if he was genuinely interested.
That's why I know it's just the Leo machismo that's acting here, I'm not taking it too personally or crushing on the guy at all... I'm a Leo too... so I get the ego boost idea lol

Thank you again Lovely Cats!