I'd write/email him saying something like this:
"Hello poppijane'sEX
These are the problems we are having with DS and school. DESCRIBE ALL
This is what we/I have already done to try and solve the problems. DESCRIBE and talk about what contact you have had with the EWO and anything they've suggested saying what happened when you tried it or why you don't believe it will work.
This is what I would like to try (home ed). However if you(ex) have any other solutions I am happy to try them first. You(ex) MUST meet with the EWO and poppijane altogether to make sure the LA know you are taking action and not just allowing DS to truant.
If ex doesn't have any alternative suggestions or if we try them for x period of time and they aren't helping you will withdraw your DS.
Explain what it involves; we will write to the school and they are responsible for telling the LA. The LA will probably get in touch and you will write back to them telling them you will be taking x weeks/months to settle down and then you will write to them about your educational provision. So you an ex should discuss what you each think are the most important things and how you want to handle home ed. How you'll make sure DS socialises and gets to experience new things. Maybe link to some of the research you've done.
What does ex think are the key things he wants DS to get from education? And what role will ex play? (cheat a bit and spell out the benefits to ex, if there are any, as well as DS - contact wont be determined by school times/holidays. Can ex share his interests with DS more?) What is he concerned about and how can you both make sure that isn't a problem?
Maybe suggest he joins this list for dads and speaks to them about any concerns. Its small and quiet - but I'm sure he'll get an answer if he posts a message.
I know that we both want the best for our lovely DS and hope we can work together on this
poppijane"
I'd think that if you give your ex every chance to be involved in the process (of trying to keep DS in school or making the choice to withdraw him) then he wont have much of a leg to stand on if he did contest it. Keep a record of all this too, so you can show you did involve him and either
tried everything ex wanted to keep DS in school
or ex didn't support you in keeping DS in school and so you had to make this decision against his wishes.