I really feel for you. This must be utterly draining, frustrating and worrying for you, at a time when you could really do with leaning on your partner to step in and help, while you get back into work, as a new mum.
Take his moods seriously, though, because it’s impacting on your son and if he’s neglecting his needs, even though it may be through ignorance, the last thing you want is for your partner’s mental health to take a dive and something happens to your DS. It sounds like he’s having early days panics. It can be terrifying being in charge of a little human life, that can’t tell you what he needs. With all the time you e spent looking after him, you probably make it look easy and your partner was surprised by how much harder it is in reality.
If you have a good relationship with your manager, tell them exactly what is happening and of your concerns, so that they know what you’re up against. Hopefully they can offer support in some way.
On the days you’re parenting together, get your partner involved in taking over the routine (after letting him know very clearly that you’re doing just that to help him get used to the routine, while he has you on hand to support, in case he needs to ask a question).
Your DS may be going through a bit of separation anxiety from you, if you’ve been the main carer right up until you returned to work. Your partner may be up against more than just the standard routine stuff, if that’s the case. His emotions and struggles shouldn’t be ignored, as it could cause him to get worse and run from the situation even more than he is. Heap praise on him when he gets it right and if he says “I can’t do this”, just reply with “it’s early days, so it’s bound to be hard as you get into a routine, in the same way starting any new job takes time to get the hang of. Keep going and run through the checklist of food/drink/nappy change/nap. Lots of cuddles, chatting to him and walks in the fresh air will help. You’ve got this”. Make it clear, you know it’s hard, but it’s just about time, and he needs to keep trying.
Don’t give him an easy out, but if he continues to push back hard, ask him to be part of the solution to finding an alternative. He should be willing to contribute more towards childcare costs, even if that means getting a second job to cover it.