The offer of more blowjobs and/or the prospect of more blowjobs would get me to agree to almost anything. Seriously, I'm not ashamed to say it. I can't imagine that if you tell him that you'd like to suck him off more and just record it, that he would be against it.
But find out if he's genuinely uncomfortable with it. Don't manipulate him into anything. Incentive is one thing. But if has a real problem with it, find out why. It may be simple nervousness. But it could be a real objection.
*edited for cleanliness*