I'm sorry that I have no proper expertise and cannot tell you anything authoritatively. I just have a few ideas for questions you could ask and avenues to explore, based on what I have "overheard" others say (and possibly misunderstood or misremembered!).
Who says your son cannot attend part-time, and why? Is there some general restriction which they believe would be breached, and if so what is it? If it's to do with him being in full-time education, then he WILL officially count as being in full-time education if you continue to HE him alongside his college course. Likewise I believe that HE young people are exempt from the government requirement that college attenders who don't already hold a GCSE C grade or higher in English and maths must work at college towards those, alongside any other college course they are doing.
Or is it just the college's judgement that full-time is in his best interests? If so, do they have access to all the relevant information which you do - bearing in mind that they know their educational setting best, but you have been your son's main/sole educator as well as his carer for the last five years?
When you say that your son "wouldn't cope" with full-time college, why is this? If you can demonstrate that it's likely to take a toll on his mental health then surely they are discriminating against him if they don't make reasonable efforts to accommodate his needs, just as they would be if he had physical health issues and could be expected to collapse physically under the strain of full-time attendance. For example, I hope they wouldn't say to a young person in the throes of chronic fatigue syndrome that he has to attend full-time or not at all.
Maybe you can press them to be very specific (in writing) about their reasons for requiring full-time attendance and explain to them exactly why you are sure it won't work?
Have you posted on the Mumsnet Special Needs board? People there will have been in a similar situation, I'm sure.
Good luck tomorrow!