Kazza I am so sorry it is so hard.
I think there is lots of good advice here.
Our son came to us a year ago. He is the cutest little boy in the world and often very well behaved and quite bright BUT he has some INCREDIBLY annoying habits which totally grate on me.
Today he hit me and I was so angry I wanted to hit him back! I did not. I put my arms around him quite tightly and spoke into his ear and told him I was VERY angry at him hitting me and his punishment would be he would not be going to explore the Taekwan-do class next week. He wouold hvae to wait another week. Of course, as I coudl have predicted, he burst into tears!Later on that night I again explained to him how naughty it was to hit people but that I know he was a a good kind boy (or whatever) and that even when he was naughty and I did not like his behaviour, I still loved him.
I am saying all this because when these things happen (like him bursting into tears for virtually no reason, shouting and getting angry when the TV is switched off or when his sister 'walks too close to him' or gets dressed before him etc etc) they kill off some of my love (temporarily) for him. I think it is very important to let children know when the behaviour is unacceptable and to show that you are important too (e.g. hitting me isn't acceptable) but also to find things that build connection as much as possible.
I must admit I pretty much hate playing with my son, it's boring, and it is not fun because he gets very upset if it doesn't go the way he wants! But some things, like card games, board games and a bit of role play, are not quite so bad! Playing ball in the swimming pool is actually fun. So I need find those things that are not too bad, put my heart and soul into them for a short while and then channel him off in direction of something else. I never really had to play with my daughter (now 10, birth daughter) She is very creative and made up her own games. But we did find things to do together like making a rocket out of cereal boxes or a hamster outfit out of tissue paper (not sure we ever put it on the hamster).
So in addition to all the other good advice (and I second LOUDLY to get any help that you can, really anything that will support you or your son or anyone else in the family)....
Make a list of the games and activities that you find best
Make a list of the games and activities that he finds best
Mix it up a bit so you are both doing things you either enjoy or do not find too arduous.
I have found art and craft stuff is not too hard to do as I sometimes get into it and make whatever it is he is making 9or my dd is making) so buy enough of the kits/packs etc so you can do things too.
Keep a record of the good times, even if they are few and far between. Write down when he does anything good or fun so you can reflect on it later. And don't forget to tell him how well he got his shoes on or remembered his coat or whatever.
Take some nice photos of him and with him, and your dh and your other son. Look at them together and compliment him. the more positive stuff he hears the more chance you will hear it back.
How do you distinguish between the boys? I've got a girl and a boy so I say I've got the best, most beautiful girl in the world and the best most handsome boy etc! As you have two boys I would listen to see how other parents with two boys or two girls distinguish between them.
I guess for me I might be tempted to say something like clever artist or footballer or Taewkwan-do boy! The obvious temptation is big boy and little boy but that will soon not work as the four year old will not always want to be little boy! you could just use their names. For me 'my special/fabulous Cassidy' and 'my special/fabulous Cassius' (not their real names).
Anyway, this is way longer than intended.
I mus admit over the past year I have wondered if I have done the right thing and whether I can love him enough and all manner of very scary stuff. But I do feel that we did the right thing, we do love him and the love will grow, but we are a year on and it is only in the last few months things have got better.