As i understand it, this would depend on whether you have sole custody or shared sutody of your child. Sole custody, you should be aloowd to move abroad and only really need to advise your ex about it. He can then fight to get access to the chikd or joint custody so that you cannot go aboroad without his permission. So I would tread very carefully around the subject and see if you can hash out an agreement about visitation whilst you are away. The friendlier you are the better, but be aware that you may be required to bring the child back for visitation rights during the year ( this can be very expensive). If your ex pays maintanence a the moment I would get this court agreed so that you have the law behind you if he stops paying whilst you are away as it will be just about impossible to get this sorted if it is voluntary and you become an expat.
All in all try and stay civil and come to an amicable solution, he may be quite happy for you to go as long as he sees his child a few times a year.... especially as he has another on the way..... but then again he might not and he may want to try and stop you going. So for your childs sake try and keep it civil, I know that this can be very hard, but at least then at a later date you can prove to your child that you did all you could to keep the contact there.