It also sounds as though you do not fully understand your son's particular learning difficulties and what he can and can't do, as opposed to what he is motivated enough to do or stubbornly not wanting to do. It is very common for those with Learning Difficulties to just give up, retreat into a dream world where everything is Ok and pretend it isn't happening, every day and every piece of work is a challenge, because it takes so much more effort to achieve what others achieve easily.
I don't believe for one second your son was not capable of writing more than one sentence on christenings if what you say about levels and past attainment is true, I can believe he may have forgotten what the teacher asked him to do and the suggestions that were given about how to go about it. I can believe that he then sat there with a piece of paper in front of him and did not know where to start, I can believe it was just too intimidating and he didn't have the confidence or tools to attempt it so he gave up, did the bare minimum and went to off to do something that did not make him feel so intimidated and lacking in confidence and hoped for the best. How sad.
If you and the school allow this to continue then you are right he will leave school with no qualifications and a belief that he is no good and he may well be lucky enough to fall into a job where his talents enable him to succeed as so many did in the 70s when Dyslexia was not recognised and no help available and recover his confidence but what a waste to experience all that loss of self esteem in the meantime.
All of this is by the way personal and family experience.
However there is lots of help available and you need to ask the school for a proper assessment of the support your son needs and to explain to you what his weaknesses and strengths are and how you can support him, then you need to support them in ensuring from the home end that he realises that he can do better, it just takes effort. Dyslexia is a disability but it does not mean you can't do things, it just means it takes more effort, that you need to find ways to achieve things whilst coping with your weaknesses. Often simple things, like, if working memory is poor, making sure that you get a record of what the teacher wants you to do, whether that is by writing it down, or getting them to do it for you (and all teachers should be prepared to do that if it is an identified need. ) and is sequencing is a problem sitting down and planning what you are going to do and following that plan instead of jumping in and losing sight of what you are meant to do. It is not necessarily that time consuming but armed with knowledge and a bit of thought you can support your son through this, but not, I am sorry, by being negative. None of us are perfect, it is frustrating , and I just had to take 10 mins because my DD leaves for uni in two days time and though she has a list she is still trying to sneak off to watch crap TV rather than have to make the huge effort it requires for her to organise herself. But no point shouting, and no point letting her get away with it either......