I hear you MissyMoose
You are in a really difficult situation and you must be under a lot of stress. 
Since you asked for advice, here are some thing that can help:
Do go to the GP and pursue that assessment for your DS. This is the worst time, not knowing if he has a real problem or not. Once you know what you are dealing with, you will start to find better ways to manage him, his behaviour will get better, and it will get easier for all of you.
Do tell the GP how you are feeling yourself. You sound burnt out.
If the GP suggests a parenting group, do go along. It's very hard to think "I need to be more patient" It's easier if you have some plans ahead of time - when are you going to praise him, what behaviour are you just going to totally ignore, even writing down a list of little consequences that you can choose from when he is really naughty, what to try when he's not doing as he's told. A parenting group will help you work that out, step by step, so you'll be following your plan and you wont feel the need to shout so much. And you'll feel better too.
Don't forget to look after yourself! That's not being selfish, parenting an aggressive child is is very draining and if you are rested then you will be a better parent to both your children. If possible try to get a bit of time each day (or each week) to yourself to do something that rests and refreshes you. Can your family help out with the children so you get a regular break?
Take care. 