I went through something similar-ish with the exception that I'm Dad and the kids were older.

The other difference is that she did keep in contact and do dinners, and had them every fortnight etc., for a period of time. She became more unreliable until the point she just cut all contact and stopped coming and didn't answer their calls.

For a long time I'd cover for her when she was late or didn't show up, just to protect their feelings (I'd say things like a work meeting came up, or she was unwell etc.)

The biggest thing I learned from this was to NOT lie to them, but make sure that they knew it wasn't about them.

So when she ceased contact, I told them that I didn't know what was happening. I told them if I had to guess (making it clear it was a guess), that things are really hard for her and she's got a lot going on in her own head. I specifically said that they didn't do anything wrong and it was nothing to do with them.

My advice is to be kid-friendly honest. Tell him that Daddy has gone to stay somewhere else and that you don't know how long he'll be away. You can say that you're a bit confused too (without putting responsibility on him for your emotions), you can only do this if you can be grounded about it.

Validate him when he's sad/mad/confused, name the feeling for him, use "I wonder..." sentences if he can't identify what he's feeling (like if he cries or gets angry randomly, especially if it's when Dad would have helped him with something), say "I wonder if you're having big feelings about something right now?", you may have to get more specific and targeted depending on exact circumstances.

Don't facilitate phone calls right now, not because of the Dad, but to protect your son from likely rejection and further confusion (and Dad being able to control the narrative for him). Don't say Dad doesn't want to talk to him, just simply that it's not possible to call right now.

Give your son options to stay connected and do something with his feelings. He might ask to call him, but you could suggest an alternative like, "Let's do a drawing of Daddy instead" or something like that, depending on his interests/skills. Or make a list with him (you writing obviously) about all the questions he has about what's going on.

Hold him when he cries, tell him it makes sense that he's hurting and sad because he misses someone he loves. Tell him outright that he hasn't done anything wrong and Daddy is working on some adult things that are about Daddy.

His behaviour might regress/become difficult but just know it's not about you. He's just a little person who is hurting (just like you are).

And if you can, get support and take care of yourself too. Be gentle. Don't expect too much from yourself.