I feel for your OP, and everyone struggling on this thread.
I wonder if we're all hitting a wall this week? We had a horrendous day today with the kids and work and stress, and the strong hints that lockdown is going to stay with only a very slow lift of restrictions is clearly a way of trying to keep us complying when we are beginning to lose resolve. Unfortunately it's the feeling of not knowing when it will end, and of not being able to see the light at the end of the tunnel that compounds the feelings of stress and hopelessness.
We have 2 small children, 4 and just turned 2, and both DH and I are trying to work. Thankfully I work PT, but DH works crazy hours. I'm finding it incredibly hard to juggle everything. I absolutely screamed at my 4 year old today, he was howling too. It was such a low moment. I felt so out of control.
In your situation OP, it sounds like you're spiralling a bit at the moment. It's hard to break out of that but recognise that you are in a spiral and that it will have to run its course and you won't feel like this forever.
Realistically, you will not be in this position until September. Even if schools don't open properly until then, other options will become available much much sooner, especially for a preschooler.
Is there any possibility of your son going to your mum's during the week? Mon-Fri and staying there with her and the rest of the family then spending the weekends with you? Obviously you would miss him terribly but it's not forever and it may be better for you both trying to work, and possibly more enjoyable for him (assuming there are adults there who are able and willing to look after him and are not trying to work as well). It does mean mixing of 2 households but it's only 2 journeys per week and if everyone is limiting their exposure in other ways it's going to be minimal exposure? Just trying to think of solutions.