had to tell my son (aged 5 at time) and daughter (aged 10 at time) the same thing 18 months ago.
Please do not do what my ex wife did was "daddy does not love them any more". I moved out but it was my ex who had affair!!
I have seen a number of friends go through a similar experience and where the kids have coped best is where parents have put feelings for ex to one side and worked together to help the children.
I saw one couple explain that mummy and daddy still loved their children but mummy and daddy had decided to live in different homes.
If you can agree on contact and maintain some kind of routine ie fifty / fifty. Alternate weekends and night during week. Every sat. Being able to explain to him that both mummy and daddy want to see him as much as possible.
get him involved when he is leaving family home to visit daddy (what toys, books etc does he want to take).
Assuming he may be staying overnight at daddys let him pick some posters if he will have his own room, bedding?
My son used to get upset if he forgot his DS or could not go out on bike as it was at other house but he's come to realise that he has different toys at mine. We both like lego so he looks forward to playing that.
Try and maintain relationship with ex so that you can discuss issues (he may be stressed but being able to talk about it and work with ex together is better for child in long run).
I personally would do the talk together. So that way he can see you are both there for him.
When i talked to mediation people they said there was evidence that a child to continue to see parents talk helps. So maybe not everytime but if you can have a coffee together prior or after drop off then do it. Every separation is different and some people cannot bear to be in same room but being able to see that you both still love him will help with his security.
Don';t let him overhear you shouting or getting into arguments with ex if you can help it.
All the four, five and six years of separated / divorced parents seem to have adjusted better when they have a regular routine. Not always possible depending on working patterns.
I have also seen different couples act differently with regards to "critiquing" ex's parenting style. ie if you insist on eating at table and ex allows eating in bedroom then that can cause issues. If you insist on no TV after 6pm and ex allows TV whenever. If you have bed time by 7:30 and ex allows 9 then that will cause problems.
But don't fight over everything, compromise and discuss the real issues.
Speak to teachers and explain situation and ask them to highlight any differences in behaviour.
Keep talking to him and reassuring him. As difficult a decision that you have made he will grow up with two parents being happier apart than two sad parents being together. Going forward surely that is better for him long term seeing two happier parents. (even though you may not be happy now)
Good luck