Can my son stay overnight with partner who lives with us?

Hello!

I have a 7 year old boy with my ex, and have been with my current boyfriend for nearly four years. I live with current boyfriend and son and coparent with my ex, he sees his son every week on agreed days and all goes well.

My son and boyfriend get on really well, and are like best friends. My boyfriend is very understanding, and supportive of me co-parenting and never over steps the mark but also at the same time helps out when he can and is somewhat a fatherly figure. My ex also has a partner who I have never had a problem with. We have all met each other and get on the best we can for the sake of my son.

I am going away with work, where my son will spend one night with my boyfriend on their own. My son must have mentioned this to his dad as my ex rang me to explain that this was in fact illegal and that if Im to leave him with my boyfriend he would call the police (!!). He also said he has a solicitor and that if I do leave him overnight ‘things will get messy’ I was pretty taken aback, however I’m not too knowledgable on the law regarding something like this?

Has anyone else had this type of issue before? Can I not leave my son with my partner on their own overnight? I am in the UK.