Here we go again, a SM voicing her concerns and getting slashed as a result.
OP, as it seems to always be the case, it sounds like you've moved very fast in this relationship. I don't know at which stage you moved in with your DP, but it can't have been much before you became pregnant or you moved in very early in your relationship. Together, you already had three children who needed adjusting and now you have another child and they have to adjust to yet another change to their lives. Your children seem to be adjusting better but maybe they are older and/or their life might be more settled compared to that boy who seems to be in and out of one house than the other, trying to find his place between a mum who doesn't give him much attention and a dad who in a couple of years had to adjust to give attention to one person to suddenly 5.
However, you are where you are now and need to make it work. On one hand, you cannot change how you feel, so pretending is going to help no-one. You need to be honest with your OH. How he will react depends on how you voice things. If you make it clear that you are concerned about the impact of his son on the dynamism of the family, how it could only work if he totally blended in and for that, he would have to adjust to new rules, he is likely to be more receptive than if you even just hint that you think his child is spoilt, immature and unpleasant. It is totally reasonable that you should insist that you work through all this together before him moving in should even be considered. Also, how the child feels now (expressed a wish to move in with his dad and you) could be very different tomorrow, so again, I would think that rushing the decision could be wrong for you and your children as much as wrong for the child.
Saying all that, I do agree with the poster who said that you should consider how you would feel if your OH said that he could stand your children and think they should now go and live with their dad half of the week because it really is no different. Of course you think your children are wonderfully lovable compared to his, but your OH might not agree at all.
I get that you are worried about telling your OH how you feel if you've pretended that all was great for all this time, but if you don't, it is only going to be worse and his feelings will only be more hurt. You can work through this, find ways to compromise by which you can all avoid building resentment that will one day take over.