"There is something I haven't asked. How do you tell your child/children that mum and dad are splitting up and are not going to all be living together anymore. What things did you say, what words did you use and how did they respond and adjust. My DS is 8." when me and xh split we had the conversation relatively casually, what I mean is we were all in the lounge and me and xh had already decided that we were going to tell him that weekend as xh wanted to go to his parents for a few days in the holiday and wanted to take ds with him. By that point I was already sleeping in the spare room so we used that as an in into the conversation iyswim, and told him that "you know that I have been sleeping in the spare room, well you know that sometimes parents don't stay together, like l at school's parents aren't together any more?" and at that point ds just asked straight out whether we were splitting up and we confirmed it but also reassured him at the same time that we both still love him, and that he would still be seeing either of us whenever he wanted, would spend half his time staying with me and half with xh but that actually I would still be taking him to/from school and he would come to me after school still etc.
The reasons why aren't IMO to be shared with dc unless e.g. one is moving straight in with someone else creating a subject which can't be avoided. in our case we just said that we like each other and love ds the same but that we don't want to be married to each other any more.
He was upset but actually I stayed in the house for a further five months because of waiting for new house to go through, etc, so he had time to get used to us not being together etc, to doing things with us separately and so on, and so when me and him eventually moved here the transition wasn't too hard.
Also at the time we were fairly amicable, although we did argue over things but in general kept ds away from such things, and when I moved out we stayed amicable for a time - well really until we both met new partners at which point xh became less willing to communicate.
DS was about a month away from his tenth birthday then.
what I would say is:
If you're able to tell him together and reassure him that you both still love him and that your relationship with him won't change he can be reassured that this is about the adults and not him.
Try to work out what you want to do re access before you tell him, although things might change as time goes on (my ds was 50/50 between us but that has changed to him spending most weekdays with me and eOW with xh, although he spends nights there in the week on a more ad-hoc basis now). But in the beginning ds will benefit from being reassured that he will see you both e.g. every other weekend and some time in between (for instance, if you go for a 50/50 arrangement.
The more you can present a united front, the better it will be for him. I realise this may be difficult given your xp's reluctance to split, but it's worth pointing out that this will be in ds' best interests to deal with the split as well as possible. If need be I would suggest going to mediation to reach agreement over this before talking to ds.
Children are resilliant, it is upsetting for them but often what is the most upsetting is the way in which it is handled rather than the fact that it happens - iyswim. Overwhelmingly children want to be secure. If they know that you both love them then they will be reassured that that won't change. One of the overwhelming things I've read about is children being affected not so much by the divorce but by their parents' attitudes to one another during and after the process.
hth