'It's exhausting giving your all to someone who gives absolutely nothing back but hatred.
To be honest, I’d shorten that to “it’s exhausting giving your all to someone” irrespective of what comes back.
It feels like hatred but is actually fear. If you can keep in mind a very scared small child it’ll help it feel less personal - while the violence must feel very personal. What do you do when he kicks and hits and head butts? Can you leave his nappy off, pop him in a safe, contained place with you nearby and wait for him to come to you? If affection from you is triggering him, I’d back right off. Lots of soft verbal communication, maybe play with his toys where he can see you and let him come to you in his own time on his own terms. Chat with him as you go about the day, observe what he’s doing and talk about it. Let his see your partner being affectionate towards you, be available to offer affection but just hang back. He honestly can’t control that reaction to you - he’ll need space and time and help to figure out that you’re not a threat, and you’ll need the same.
And can the sw or foster career tell you if they’ve seen such a “fight” reaction in him before and what worked? Maybe explore other ways for him to express his “fight”, foam bats and balls, scrunching up cornflakes in a bowl or bag, messy play - I used to squirt shaving foam over the back patio doors so the could draw patterns etc, nice and sensory but easy cleaned away. I’d also explore safe holding and safe restraint for the times you simply needs to put hands on him (eg coat, car seat etc).
I did have a massive episode of rejection with my DD after contact with her foster caters, which was very hard because she openly talked about hating me, hating our house, asking to leave, not allowing me to do anything to care for her while also literally being within touching distance at all times - for 3 months.
And to go back to the start - you can’t give him your all, you’ll have nothing left for yourself. What are you doing to promote your own resilience during this really traumatic time for you, what supports do you have - formal or informal - what time out do you get, what resources can you access to help you cope with something that sounds incredibly hard. If the answers to those questions are “nothing much, I can’t really get away, feel guilty, no one gets it”, think of ways to get support. I used adoption allowance to see a therapist for the first 18 months or so - at times having that space to really speak freely stopped the adoption breaking down, and also helped to flag when things really really weren’t ok and when I needed additional support.
You’re not meant to know how to do this, you’re a very very new mum, at this stage following pregnancy you’d be able to at least leave your little one in a Moses basket and pee in peace - and you’d have lots of practical advice, newly delivered mummy friends and people wanting to hold baby for you. You likely have little to none of that, because people forget and don’t understand adoption - did you even get “new mummy” flowers or cards? Not that that makes it easier, but it does actively remind you that you’re completely new to all of this. Go gently with yourself - if you can be compassionate to yourself in it, you’ll have more head and heart space to bond and care for him.
Apologies for the length of post - I just remember how very hard the early days are.