Sounds like you're new to BB. First time I BB'd a bottom same thing, came within first 5 strokes of him sitting up and down on my cock. Second time, similar thing. In both cases, was not expecting the bottom to ride me BB ended being a pleasant surprise.
A couple things I find, I try not to do a lot of stroking before fucking the guy. Just enough to keep the cock hard while working him up. When I get in I pump a couple times and if I feel like it's going to blow I stop while all the way in. I find when I pull back to the sphincter or out, he's usually tighter there and if he's a cum hungry bottom that starts squeezing (like I do when on the bottom) it can end then and there. Sometimes I do pull it all the way out too and give it a few seconds to calm down before thrusting back in, only issue with this is if you like the feeling of penetrating that hole, then you could have an issue cumming again.
Then there is always the, can you stay hard after you cum situation. Other night bred a guy and came pretty fast, was so horned I didn't soften so I kept going and gave him a second load. After blowing the first load, the hair trigger is off. Best part is your cumdump will be in pig heaven when you tell him he has two of your loads up there.