have his mental health issues been diagnosed?
anyway... had /have simialr situation...
so, he isnt going to change - unless he is sekeing appropriate help for the metnal ehalth issues. even if he is, it will be a long haul. and in my experience - good days cna be followed b y bad - good weeks can be followed by bad weeks (depending on diagnosis and treatment being given of course)
next q - are you separated? is it over or not? you say he has threatened to end the marriage? (my exP made those threats too -funnily enough when i did properly end the relationship he couldnt accept that... )
maybe at least for now you ahve to be clear to yourself, to your H to your son: foir now (next few months) it is over, and he has time alone to sort out his mental ehalth issues,
then you need to think practical: finance.
and contact with his son - regular, set days not as and when your H feels like it. it will help your H to know on xx days at xx tiems he will se ehis son .
if you want you cna slo set set times time-limited where you enet with him to discuss your and his issues, your relationship etc.
the only way to deal with MH issues is to set clear boundaries and stick to them. dont have him in your house for now, you need some space; have your son visit with him at his parents place or public place eg soft play/ park.
make that separation adn set specific times to meet/speak - that way if there is a future back together you can set the terms and conditions and rules of behaviour. the threats adn damge --are not explained by mental health i dont think - it gets tangled up but if he is using threats of suicide as threats then he knows what he is doing..... if he is truly psychotic/oaranoid/cant control himself then he needs erious treatment/medication and that si not somehting you can provide unless you a) professional MH worker and b) dont have to look after your son at same time.
i dont think anyone can look after a small child and a seriously ill adult at the same time..... child must come first . if you addressing the adult's issues then someone else must be looking after the child...that is by the by as clearly you are with your child and your H is at his aprents. decision been made.
for you son, if your H is going to stay with his parents for now - then establish a set routine and schedule for contact so you can say and show on calendar - (you can make a simple days of the week wall chart) you will see daddy on this day at this time at this place. maybe at his parents place? so they around and you are not?
you cannot apply normal rules of amicable separation with someone who doesnt act "normally".
you need to disengage from him, put you and son first, establish regular routine contact dad/son that does not involve you.
is easier said than done...i know. i amde many mistakes such as allowing exP to visit dcs at my new place etc. thinking i could supervise contact etc.
are you working? what is your financial position?
what are you asking solicitor to say? if anythign like my ex he will not react in "normal" way to any solicitor letter...
it's ok to put yourself and son first and create stable secure safe home for you and him - and leave your H to get help etc. if his parents blank you - well their loss - but you could write to them to ask for help in supporting contact with their grandson and son...
. so long as you have rl support yourself.
what is SS suggesting?
if you had to call police and he has damaged stuff then is best you don't have him back (not even to talk - you can do this elsewhere in public place if you want to for set limited time eg one hour) for some time - it will just kick off again; if he has MH issues then many months needed til you can be sure he stable...
the worst times for me and dcs were the bit when i adh moved out with dcs but was trying to estbalish amicable "you can come see kids when you want" - it jsut didnt work he would kick off damage stuff, threten suicide etc if we didnt get abck togetehr...
the best time was after i adh called police, banne dhim from home and gradaully it led to very set routine for (initially contact centre) supervised contact...and for myself trying to disengage and trying to only communicate by email...
you stillc alling him H so i guess is not o clear for you - but maybe by establishing clear "separation" from now (even if with view that in few months you might wish to get back together again - as it not clear what you want in terms of separating ? ) and taking seriously his threats/MH issues and setting very clear boundaries and set routines/times for contact with his son.
this will actually help him.
one thing family therapist/psychologists told me was i was sending mixed messages eg by allowing him free acess in my home to see dcs etc - for a person without MH isues it might have been clear - for him it was not - he started saying things like "but we live together" when clearly we did not! but as he was allowed in my new home he thought it was his because his dcs lived there..