To some extent, let your DS dictate the depth of conversation. He may be reluctant to confide much at first, but if so that should improve with time.
My dc appreciate honesty, I've found that telling them that I will always do my best, but sometimes even grownups get things wrong helps. Admitting that I have handled a situation badly has never had a negative effect on my dc's opinions of me, what it does is model the behaviour of admitting to errors, correcting them where possible and learning from them. DH and I also have a policy of 'you will never be punished for what you feel, only for what you express badly' so:
A discussion or journal entry (ds used to find this easier) saying that dc1 'hated' dc2, wished dc2 had never been born and that dc2 had destroyed dc1's life would incur neither correction nor sanction. What it would do would be open a dialogue so I could find out why dc1 was feeling that way - dc1 has the right to feel whatever they are feeling - so that I could try and help.
Alternatively, dc1 venting their frustration by hitting out would incur an immediate sanction for the behaviour - and the reasons would be discussed once everyone was calm.
(the example above could easily be re-worded with you as the target of his dislike either, depending on what he's feeling)
To use an example from your OP:
Any slight tip or advice and feels criticized.
I'd find a time when I could sit/cuddle with him so that he was the sole focus of your attention and gently ask him about it. Tell him you're worried that he seems to feel picked on and criticized and ask him if that is actually the case. Reassure him that he won't get into trouble for being honest and that you really need to know how he feels so you can help. It may take some time, but if he gives you an indication that he's interpreting your word/ actions negatively then your response is a huge hug and THANK YOU to him for telling you. You're sorry it's come across that way and you didn't mean it like that - keep the apology for when you identify something you realise you've done wrong, that way it means something definitive. Then ask him what he thinks will help - he might not know - but try and find out what it is about the situation that makes it feel bad (timing/tone/feels like its about you or his sibling rather than helping him/etc). Lots of hugs and reassurance that mum is going to work on it and that if he's feeling bad again to let you know.
Sometimes with my DS a whole tsunami of issues have come rolling out in one go and the only way I've been able to deal with it is to acknowledge it at the time, hugs etc and then talk about it more in the following days (having discussed what he has said with DH so that we can work out together what our approach should be) - this is because my DS's emotion dumps tend to happen at 9pm or there abouts!
The biggest change I think you may need to make (just from reading your posts) is to remember that you DS1 is only 8, if you think a lot of what you're saying is going over his head you have almost certainly diagnosed your own problem. Get to know your son as he is and set your expectations accordingly. Oh and find some way of keeping the lines of communication open, so he knows he can talk to you without being bushed off in favour of the younger one's needs/wants/tantrums. It's not easy and changing the way you interact may well take time (and you may backslide), but he will almost certainly appreciate the effort you make.
Good luck...and well done if you've made it through my ramble and got anything useful from it.