Where are you in the EHCP process? Is the LA sticking to the timescales?
Unless you have to appeal, and I appreciate you may, potentially more than once, it won’t take 18 months for an EHCP. There are statutory timescales. LAs like to think they don’t have to adhere to these, but they aren’t optional and parents can take enforcement action.
If the LA refuse to assess or refuse to issue, you should appeal. You can get an EHCP for the needs you describe. You don’t need EP and CP reports beforehand. They can be part of the EHCNA.
With an EHCP, there is EOTAS - Education Otherwise Than At School. You will also see this called EOTIS, with the I standing for In. And you will sometimes see it with a C on the end for College. This is a bespoke package of education. Education in the broad sense including things like mentoring and therapeutic input. Not just academics.
There are options for the academic side of things. As well as online schooling, other options include online tuition (individual or small group), pre-recorded lessons or F2F tuition at home. Then, once DS’s anxiety improves, there are tuition centres and alternative provisions that might work for him, or there may be a special sit placement suitable within travelling distance once DS’s MH improves.
Exams can be sat at home if necessary too. Or certain exams can be remotely invigilated.
Without an EHCP, whilst the LA has the power to arrange alternative provision for post 16 students unable to attend college etc., they don’t have a duty to like they do for compulsory school age pupils. So, whilst you should ask the LA to arrange provision, most of the time LAs don’t/won’t.