Why does his mum never see him? Is this something that could change in the future? Do you think your ss is confused by his mother being in contact but not actually seeing him? It sounds like he’s confused by his role in the family. Can you speak to her about working with you instead of against you for his best interests?
Do you think he loves looking after the baby or is it just that it makes him feel important and needed? There is a difference between the two and by the way you mentioned him ramming the bottle in her mouth I suspect it’s his way of more him trying to assert authority over you then wanting to care for her.
If he sees you as an equal or a rival it’s likely he feels he has as much right to care for the baby as you - in the situations you describe it seems he thinks he is better at caring for her then you!
You must have the patience of a saint to put up with this. I think it’s admirable that you haven’t lost your temper or showed your frustration, it’s obvious from your posts you are doing your best to make this work but in letting him take the baby whenever he wants you are giving him too much control over you. He needs to see you as a parent figure and it sounds like he feels he’s the one parenting! You can already spot this but does your husband?
I have a lot of respect for anyone who is a step parent, it’s something most people would struggle with. Your example with the ice cream shows how different the bond is with your own child and a step child, with your own it’s instinctive but you have to work at loving someone else’s. I’m sure you would be happy to share an ice cream with your own child, It’s only natural to feel that way.
It’s like step parents who struggle with step kids getting in their bed in the night or morning, it doesn’t feel comfortable for a lot of people to share a bed with a child that’s not your own, no matter how strong the bong between you is.
Your husband needs to back you up and help his son see you as a figure of authority and not a rival and to help him understand he’s the babies sibling and not another parent. If your ss starts trying to take over he should step in and tell him that you are fine managing on your own and are perfectly capable of managing yourself.
If you choose when he can help rather than him taking over then he might have a better understanding of his role as a brother not 3rd parent.
You have alluded a few times that some of the issues come from your ss relationship with his mum, is this something that can be addressed and worked on?
I hope you and your husband can create better boundaries and help your ss understand and stick to them. Can you give him any other tasks or responsibilities that don’t involve actually holding the baby but make him feel important? If they have to involve the baby then maybe something like choosing her clothes for the day or organising her toys? That way he feels involved without the risk of accidentally hurting her.
When your ss is claiming to know everything like your example with the boats you could try asking how he knows that? This would drive me mad, I’d probably start acting like a child saying “how do you know? Prove it!” Which is why I’ll never be in a relationship with a man with kids 😂.
A pp mentioned how this situation can turn nasty if jealousy continues building and mentioned the possibility of your ss starting to harm the baby.
I do think this might be something you need to watch for. My friends son used to attempt to take care of his baby sister and tried to take over everything which we assume was for praise and attention. When his efforts started to be prevented instead of rewarded he started feeling resentment towards his sister and my friend realised her son had started trying to hurt the baby by nipping her, pushing her over and on one occasion she caught him trying to feed her whole grapes because he’d heard my friend talking about needing them cut up as they were a choking hazard.
He's seeing a counsellor now as he had other issues but when I saw that had been mentioned by the pp it made me think it might be a common issue.
I hope you can work things out and you and your husband can work together on this. I do think you are amazing though for the way you are handling such a difficult situation with such patience and understanding.