It's only 'amicable' because the OP isn't willing to rock the boat.
Amicable should ideally be a two way street and it needs to be founded on deep mutual respect and trust.
When a situation develops where one ex spouse asks for an arrangement that is not acceptable and the other former spouse feels nervous about objecting to it because of fear of causing bad feeling, then there is a power imbalance that is fundamental to the relationship instead of the mutual respect and trust.
I am fearful that if I fight for that extra time now it will cause hostility and that is definitely not the best for ds. So far everything has been negotiated with us alone and wishing to keep it that way.
The OP herself has no intention of causing hostility and is not motivated by hostility.
It is the choice of her exH to take umbrage, to respond with hostility, to create a coolness or to react with anger.
It is the choice of the exH to stay in a career where he can be posted for long stretches.
It is not the role of the child's mother to enable the choices of this man regardless of her own feelings or misgivings about the welfare of her child. The role of mollifier of a man who might turn nasty is one that needs to be rejected. 'Amicable' crosses a line into 'fearful' when that role is present.
Sometimes digging in your heels and showing that you are not a pushover is necessary in order to prevent any further unreasonable requests. You have to teach people how to treat you because some people are inclined to keep on pushing to see what they can get away with.
OP, please get legal advice, and be willing to respond with a statement based on legal advice. It is your exH's choice whether this gets nasty. You are not responsible for other adults' reactions to your decisions.