I am so sorry your ds is going through this. It sounds very similar to what my ds went through when he went into year 3, right down to the toilet incidents. He is 9 now and has finally learned to use the fly on his trousers, but its taken a while for him to master it. We had the same problem with him not being able to get to a teacher for help when bullied in the playground as well and incidents that were happening out of the line of sight of the playground supervisors. The school has now changed the positions where they stand so that they have a better view of the whole are and have eliminated the blind spots. What also helped, was giving ds a couple of 'designated dinner ladies' who were informed of his difficulties, that he was being bullied and who the potential bullies were, so that they could keep an eye on him.
Your ds sounds very similar to mine at the same age, in terms of anxieties around school and bullying, fine motor and handwriting issues. I totally understand how you are feeling - its heartbreaking to see all this happening to your child and feel so helpless, but ... it can get better I promise. You need to get your great big steel-capped school-butt kicking boots on and make it happen. Don't worry if you end up being unpopular with the school/staff - its kind of inevitable really - our Head literally runs away when he sees me across the playground these days! 
My ds is 9 now, in year 5 and a million miles away from the scared and anxious little boy he was at age 7. He still has his problems and strugglesm, but the bullies have given up, he even went away on an outward bound weekend with the school last year and managed a year of swimming lessons, despite having sensory issues around water on his face - more importantly he is happy and that's the most important thing.
As others have said - and I apologise if I'm repeating anything, as I haven't read the whole thread word for word yet - you need to get the bullying stamped on straight away - that's what's eating his confidence and causing the most anxiety.
First of all, you need to write down a list of all he incidents, big and small. Arrange a meeting with the Head and supply him with a copy of the list. You also need a copy of the school's Anti-bullying policy, which they are obliged to provide you with, if its not on their website, the office has to provide you with a hard copy. Take this with you to the meeting as well and highlight everywhere they are failing to carry it out. (Our Head ended up agreeing that ours wasn't worth the paper it was written on and it has since been rewritten.)
The anti-bullying website Attila linked to is good. There are some others as well, so have a quick google and get some really salient quotes about protecting the victims of bullying to reel off as well.
Ask what the consequences are for bullies in that school and what will happen ever time there is an incident and also how this will be taken further with the 'bullies' if the first line consequences don't stop them.
Ask if there is any way he could have a job inside school in the short-term, maybe tidying the library books or something. My ds was allowed to sit in reception and read if he was too scared to go outside and this broke the bullies focus on him as a potential victim.
Another really useful thing would be from now on to keep a daily diary of everything that happens at school, every bully incident and anything else your ds tells you about his life at school, plus anything that your dd can add insight to as well. Doing this really helped us, especially when the school realised we could back up every complaint about bullying with dates, times, details etc.
Finally, the fact that he doesn't have a diagnosis is neither here nor there, the school should be identifying his difficulties and supporting them appropriately. He doesn't need to be failing academically to get a statement either - although they may well try and feed you that line. We are just going for a statement for my ds and its essentially to support his social and communication difficulties rather than his academic skills. The inclusion team locally call this an ASD Statement and they are apparently becoming more and more common as schools and LEAs realise that social and communication skills are a very real barrier to learning for some children.