I am after some advice on custody arrangements son. He is…

I am after some advice on custody arrangements for my son. He is 2.5 years old. I currently have him 4 - 5 nights per week and his father has him 2 - 3 nights per week (however I have worked the arrangement so that my son gets almost even amount of 'waking
hours' with us, but can still keep him as settled as possible in the one house at night time. Every time something happens in my life that makes me happy (eg new job, new partner etc) it ends with my ex partner wanting to change custody arrangements because
apparently what we have (which is quite a flexible arrangement) is unfair. I have a partner who I have been seeing for a while now and is a big part of my life. I would like to know what opinions my ex can have on this relationship and where laws/rules lie
in terms of my new partner moving in with us. Obviously I would like to do things the 'right way' by my ex and let him be aware of what is going on in mine and my sons life, however if he is going to react this way every time I am unsure of whether I actually
need to tell him or whether it is best to leave it and make decisions on what is best for my son. Do I need to tell my ex that my new partner is moving in? Can he have a say in this? Can he force me to change custody arrangements because someone has moved
in with me? To put this into context a little more, I would definitely not bring anyone into my home that I thought would be a negative influence on my son. I also don't agree with the people who have lived with my ex and my son (when he is at Dad's house)
but have never made it an issue because I understand that his Dad needs assistance financially in paying the rent and would also hope that his Dad at least trusts the person who is living with them. Thank you for your feedback.