If you have access to a second helper, have the first helper remain still and dull while your dog is on the sleeve and you are ready to out your dog. I don't know if you use a whip, but if you do, have the second helper come out on the field in the dog's field of vision and start cracking the whip and making prey movements with the sleeve. He can move closer toward your dog if he still doesn't out. The goal is for your dog to see the second helper as more interesting due to him stimulating his prey drive while the first helper is essentially flat and just standing there. The first helper can also clip a leash to the sleeve and take his arm out and back up, but still keep tension on the sleeve. If your dog does out the reward is the bite on the second helper. If that works, I would have the second helper slip the sleeve right after the dog targets.
If that doesn't work, you can just wait the dog out, while the helper is passive. This could take several minutes and you would have to give him a reward of rebiting the helper. Another option is to flank your dog to out him. It doesn't have to be harsh, but quick and enough to surprise/shock him off the bite. Again, he should be rewarded for outing. If you are worried about getting bit, wear leather gloves. Or you could choke your dog off the bite by grabbing his collar and lifting his front feet off the ground and waiting for him to out while giving the command. You are not really choking the dog. Again reward him. If you dog really enjoys the fight, getting to out for a tug can be difficult.