I think that you have to be very clear as to what is good behaviour and what is bad. Praise, praise, praise the good behaviour - even the tiniest thing to start with.
With the bad behaviour you have to set very clear parameters and STICK TO THEM!!! When DS1 was 18 months - 2 years I cleared the playpen and it became the 'naughty spot' (I was too nervous that he would not sit on the step!). You then have to pick your battles but be very clear that hurting his brother is not allowed and he will have to go in the pen. For all other poor behaviour, give a clear warning that 'throwing food on the floor' is not allowed and if he does it, he will go in the pen. If he does it, put him in the pen for 2 minutes (a la Super nanny). During the 2 minutes, ignore him. Afterwards, explain why you put him there and then get him to say sorry and have a hug. If you are consistent, he will get the message!
When out and about, the sanction for my DS was that he would have time out in the buggy. If he ignored the warning, he was strapped in for 2 mins etc.
Now he is 2.5, we still use the naughty step (pen now gone!) but it is so infrequent as normally the warning is enough for him to stop. There is no buggy seat for him anymore, so the sanction is loss of tv time, no story, no ice-cream etc depending on where we are or what we are doing. But the key is that you HAVE to follow through. So my DH is completely unhelpful as he will say things like, 'if you do that again, we won't go to the park" - which then backfires as we all want to get out of the house - so be careful as to what you sanction!!!
It will get better but most of all he needs to understand that he is lucky to have a little brother and that he is a big brother. I also second that you might want to spend some special one on one time with him so that he feels special - it is important that you both get to have time to strengthen your relationship rather than you feeling cross with him, and him feeling shouted at!
Good luck!