Test the water sample that was in the bag if you have any water left. (this should be done IMO
before starting the acclimation process as that will give you a good idea of how long it should take).
Other then that, was there something in the other tank that could have stressed it out? Chasing it with a net? Don't use additives, that will add to your problem. Also, you say you are acclimating him to your tank..a qtine tank or your main? If it is possibly ill, I sure wouldnt be putting it in the main tank.