What should I do?

My girlfriends sister has a staffy and Dragon met him a few times and he's always very excited when meeting him, his tail wags like no tomorrow and he starts to "cry", as I call it, to play and most of you know that Dragon is aggressive, well can be he's been ok with 2 dogs in total of the lead and he was superb, always the one running away instead of being the dominante one... My problem is he always tries to hump (which is the only thing that he does that is dominante other than that he's the submissive one) most dogs don't like to be humped, and my girlfriends sisters staffy (Kano) is the same, he is always super excited and very over the top . My boy went to jump on him once luckily they were both on the lead and we just pulled them apart but Kano turned back to try bite him...

Kano is very good with dogs but has gotten in a few fights before when a dog annoys him to much, which makes me scared of letting my boy of the lead with him... I'm nearly 100% sure that Dragon would play with him, my only fear is if he tries to hump Kano and a fight happens...

How could I in a way deal with this? Even if my boy was friendly and so was Kano a big fight could happen if my boy humped him, so how would I deal with it and kinda teach Dragon not to hump? I know to separate them and leave Dragon in another room for a few minutes but by the time I go to get Dragon one of them would already of bite the other, and if my boy bites Kano first then it will be very hard to separate them as he just never let's go... Not sure if Kano would be the same but I don't want to risk it and get one of the dogs hurt, so how could I show Dragon not to hump while they're not of the lead?

I was thinking of going closer to Kano wait for him to try to hump Kano and then pull him away say no walk away for a while and come back until he stops and repeat this for a week or so?

personally if it was me i would'nt even entertain it,walk them on the lead together but if i knew my dog was da and she is i just would'nt,if you can stop dragon's humping then may be but if there is any risk of a fight breaking out dont take the risk.

As lovely as it would be for them to both get on, I also wouldn't even risk it. Sorry! When bull breeds don't get on the stakes are high, you've worked so hard to get Dragon more socialized there's no point in setting him back!

Don`t give up yet.
I would walk them on the lead together as often as possible so they
get accustomed to each other. When you wan to chance them off the lead
you could muzzle both of them so they can`t do damage to each other
if they decide to fight.

Mounting is not always a sign of dominance.