You can pay for a private assessment mate, I don’t know how much they are but probably not cheap.
However, they are normally pretty understanding about these things and you will have a bit of time to save up before you get an appointment.
What you can ask for is an opinion letter from your point of contact at CHAMS, explain that your lad can’t access the help without it and that may do the trick.
Edited to add: That’s what we did while we wait the 2 years for a proper assessment, even the private bloke is booked up for 12 months !