School cannot meet needs of child but do not yet have an EHCP - what happens next?

Our son (6) is in a wonderful mainstream school. He has a diagnosis of a rare genetic condition that is linked with behavioural issues and/or learning disabilities. He clearly has severe behavioural issues but no formal assessment yet. The school have done everything they can to support him including almost full time 1-2-1 support. We were just about to start the EHCP process but his behaviour has deteriorated massively the last month. He is increasingly violent towards other children and adults, including his support person. He is spending 90% of his time outside the classroom. He has now begun trying to escape the building and has almost managed on a few occasions. He is now a health and safety risk for himself and others.

The school tell me they have run out of options and can no longer manage him with their current resources. We are having an educational phychologist assessment on Thursday but clearly we can’t wait a year or more for the EHCP process to run its course. Sorry for the long winded context but my basic question is, in situations like this is there any sort of emergency support or is an EHCP expedited? He’s a danger to himself and others in the present situation. There is a specialist school nearby but I’m unclear if we can approach them without an EHCP. Any advice on what to ask/push for/ do would be really appreciated.