6 month old has serious mood swings...either really happy or absolutely miserable, HELP

I’m seriously at my wits end. I love my baby so much it hurts but he is so difficult. It’s like be only has two settings - incredibly happy or absolutely miserable. Everyone we come in contact with comments on how happy he is and that he’s such a good baby but if only they knew. People see him in such good moods that they don’t believe me when I say he’s a nightmare at home. When he’s happy all you have to do is look at him and he smiles and laughs at you. Tickling him has him in fits of giggles. He screeches with happiness and bounces up and down. But when he isn’t happy it’s just constant moaning and whinging and for seemingly no reason. He’s had feeding issues since he was small but no-one can figure out why. All health visitors and doctors say is ‘I don’t know’ and ‘that is just who he is’. He’s ridiculously fussy with them. He’ll demand one every 3 hours but take up to 2 hours to actually drink it. He used to cry during them but now he just fusses and stops drinking every 2 seconds but then whinges for it back and it just goes on and on. We’ve tried gaviscon and ranitidine but they didn’t help. He’ll be sat on your lap fine and then all of a sudden have an outburst of whinging and he’ll start fidgeting and kicking (to the point I have bruises) and pulling my hair. He doesn’t last in his activity centre or play mat for long before the whinging starts. He is teething which doesn’t help but he’s been like this forever. He can roll and sit up. He doesn’t sleep through the night anymore but he used to. He just wants to be doing things constantly, he never stays still. I look at other babies at baby group and they’re happy and content and just chilled out but mine is just constantly on the go. Now I’m not complaining about that, I could just do without all of the moaning and fussing every single day for seemingly no reason. Nappy is clean, he’s been fed (or tried to but refused it), he isn’t hot or cold, isn’t tired (although he does fight naps), can’t be bored with his mountains of toys. I don’t know what to do with him. I’m always hesitant to go out anywhere because he’ll want a bottle and just fuss around with it or he’ll just whinge unless we’re around people that he can get attention from. I really don’t know what to do, can anyone help?😩