Our 9 month old Tibetan has started this awful high pitch screaming to get attention. Any ideas on how to stop it??
We have tried ignoring it so as to not let him think it will get our attention be when he’s at the back door it disturbs the neighbours. Someone called the RSPCA as it sounds like he’s being murdered.
Any help greatly appreciated.

All I can offer is you be out there WITH him to get him indoors as soon as he's done out there.

Hopefully Nikita or someone with similar behavioural experience will be able to advise.
However if this is when he goes outside without you I agree that u need to go out too and bring him straight back in . Using treats when he isn't making a noise so he begins to get the idea that no noise equals treats. If it suits way to ask to go outside then wait until you have a gap in the noise and treat and let him out when quiet. This maybe very brief at the beginning

I think we need a bit more info on the situations where he is screaming for attention. Where is this happening? At home? In particular rooms? And in what context? Waiting for meals...?

Some context would definitely be useful, here.  If it's only at the back door, I'd stay out there with him.  I'd do that anyway, with many (if not most) stolen dogs being taken from back gardens.