I agree with the others that you are doing a brilliant job. It sounds tough. I don't know that is be handling it as well as you.

As others have also said, I get the vibe that you feel his behaviours are outside what is classed as 'normal'. If that's the case is definitely get him seen to, that way you can either rule that out or, if there is something to it, get some help.

In the meantime, would any of these things help:
1. Doing 'first' and then 'then' sentences with him, eg, "first we are going to play blocks and then Mummy is going to ....". It could even help to explain these things visually to him (eg have a card that you can Velcro little cards with these activities to - that way he can 'see' what's going to happen.
2. Getting an egg timer (or some other sort) and telling him that when it is finished you'll be back to play (you could slowly increase the time).
3. Pump him up with heaps of praise whenever he does play alone or doesn't have a tantrum ect (DD1 loves hearing how happy Mummy and Daddy are when she displays a behaviour we love).

A question - what's his language like? Is it possible the screaming could be due to not being able to 'use his words' and express himself any other way?