Suzza no doubt more knowledgeable people will be along in a minute but I just wanted to say that my son started full-time last half-term and we have had some problems with aggression which now seem to be improving. I am 
to hear that the school are talking about excluding a child with special needs at 5! At my son's school they talked with me and tried different strategies out to help him control the urges. They asked me to keep a record of when he gets aggressive at home to try to work out what is triggering the behaviour.
In my ds' case I noticed it is much worse when his blood sugar drops as he cannot eat even when hungry when he feels stressed/overloaded (I have posted about this elsewhere and it seems my ds is not the only one like this!) The school agreed for me to include breadsticks when the children have an afternoon snack of fruit. We have also been encouraging him to clap when he feels the urge to lash out. ds cannot control himself properly when his blood sugar is low and nor can he eat enough in one go in a school environment to last him with only a fruit snack so we argued an exception could be made on the grounds that it was a medical need.
So if you can get the school to be specific about what behaviours are the problem and what times they observe them you might be able to find out the solution together. It is quite unfair to be putting the blame on your son, as you know his behaviour is a sign of distress of some kind and tbh most 5 year olds are not fully in control of their behaviour and have to be helped by supportive routines let alone ones with special needs.
If they exclude him would the LEA not have to think about sending him to a special school? on the grounds that the school he is at had shown they did not have what it takes to accommodate him?
Of course you are upset when your boy is struggling, that doesn't mean you are weak, when our children are unhappy we feel it ourselves don't we? Maybe your husband is struggling with his own sad feelings and trying to shut them out?
On this board we are all in this for the long haul and understand those days when everything seems so sad and overwhelming. Parenting a child with sn is one obstacle after another but the good news is that this board is full of examples where obstacles get overcome, solutions found, progress made and oh, the pride when your child achieves something!
Every bit you overcome even if it is something small makes you that bit stronger - and it's the same for your lovely wee boy. At least he has a loving home to come home to each day and that gives children the strength to overcome an awful lot. :)