Some children need a lot of warning that a change is going to happen and others just use the warnings to get more anxious about the impending change so it makes it worse. What you may have to do is put out what you need for the next activity and then wait for him to show interest in it and make the transition himself. For things like coming for lunch or getting a diaper changed I would just pick him up and take him to the next thing and just keep repeating "it is time for .... now" which lets him know what is going to happen, he has your physical support while transitioning and he is also learning that no matter how much he protests the change is still going to happen.