ds1 was 3 when exh and i separated. inlaws told ds1 within a day or 2 that daddy needed to work away from home so that's why he didn't live with us. very unhelpful and made it harder for me to explain.
i would say tell them as much truth as they can handle.
i told ds, at the time, that sometimes mums and dads find they are not very happy living together and so decide to live apart. i pointed out that we don't live with his grandparents but they still love him, that we don't live with his uncles but they still love him, etc., and that it would be just the same with his dad.
4 years on (and us living with someone else and having a baby together) ds1 still asks sometimes, and as he gets older i find it even more important to be very matter of fact about it, stressing that we all love him (new step dad and new step mum included), and he seems to be pretty ok with it.
but realise it may be harder with older kids, and with the particular issues you face iwth your boys.
I didn't see this. And my dd is only 1 1/2, so any factual knowledge I have is useless.
It must be so hard. Even my little one asks questions in her own litle way (Daddy? Daddy?) as in, where is he gone??
I think honesty as they can handle seems to stand the test of time, regardless of intelligence or special needs, or, well, anything, really. And I hope that things settle and your ex dosn't keep changing things, so that their new "norm" becomes something they can live with.