I'm a bit worried about DS1 (4.5 years). He's always been impatient and got frustrated easily.If he's playing with a toy, and something doesn't work, he'd get instantly upset and throw it away instead of trying to work out how to do it. DS2 is quite different in nature and can patiently work on something. I'm worried that DS1 will have trouble learning if he's not helped to overcome this fast temper.

Also he doesn't like to be 'helped', and if I try to correct him (e.g. holding a pencil), he insists he's right and gets angry at me.

Any suggestions? I will probably go and see a children's psychologist for tips but can't for a few months.

I don't really have any sugestions but to say that maybe talking to your GP first about it and they can reccomend a specialist.
My DS1 is like this occassionly when he can't work things out he gets very angry and frustrated. WE try to calm him down and then get him to look at hte situation and see if he can work it out himself. That tends to work for us.

Good luck

DS (3.5y) gets annoyed if a toy isn't working properly but that's usually when he's tired. We calmly remind him to either try to solve the problem on his own or he can ask us for help.

With the pencil grip, I'd let it go. DS has a poor pencil grip right now and when I've tried to correct it he insists his way is the right way. So I let it go because I wouldn't like anyone correcting me and pointing out my mistakes. Also, I have found that DS responds well to his teachers when it comes to things I've been trying to 'correct' for ages. An example is his writing, he started writing letters in a lazyish way whereas he had always formed them properly. All it took was one lesson with his teacher and bang, he started writing the letters properly again. In our case a change of scenery, so to speak, has helped.