My ex partner has asked for full custody

[Lovecats27]

As title says. Myself and my ex partner have split up over a year ago. Since then we had 50 50 childcare agreement for our 4 year old DS.
I have received today an email from his solicitor saying due to my work commitments he wants our DS to live with him full time. I work as a nurse 3 long days a week 8 to 8. He is trying to use my work against me. I'm terrified he will take my son away from me. As is the weekend I cannot even contact a solicitor for help.
Our relationship was hell, including him getting arrested for domestic violence before our son was born and me stupidly enough not pressing charges against because I wanted to give him a second chance. I recently found an air tag in my car under the child seat that my ex put there so he can track all my movements. He even had the cheek to come to my door and ask for it back when he realised I found it and took it out. I don't really know what to do. I'm desperate. Should I press charges against him for putting an air tag in my car? Would that help if he was to take me to court for custody?
If anyone can please advise me. I don't want to lose my son. I think he is only doing this because I have a new partner and he wants to continue to manipulate and control everything and so far he seems to be winning.

@Lovecats27 Absolutely. He warned.me that if I ever was dare to leave him I would lose everything. He pretty much succeeded in that.

Please speak to police. If there's recorded DV you won't have to do mediation and might qualify for legal aid if you require it.

He can ask for full custody as much as he wants, but his reasoning is ridiculous.

You have a job. Like most mothers do. You work full time just like everyone else, but your hours split in 3 days rather than 5 days.
As long as you have childcare (nursery, nanny, grandparents, frien, etc) and your child is safe and happy during your work hours, there is no way he has a case here. No judge would ever take a child from a mother who has a decent job based on this reasoning - especially if you consider his past behaviour towards you.
Can he prove in any ways that the child is been neglected or harmed during your work hours?

If you are in the UK don't worry 50/50 is the preferred option for court.

3 shifts is normal in my opinion..the fact he's put an air tag under your child's car set is very concerning and stalking is taken very seriously I definitely would report that to your solicitor or the police ..so sorry your going through this , I hope you get it all sorted

Thank you all for your replies and kind words.The 50 50 agreement would have been that he has our son on my working days and I have him on my days off however he seems to think he can use it against me that I work 3 long days per week to say im an unfit mother and he can do better than me because he can adjust his hours according to our son' school schedule

Parent alienating comes to mind...I wouldn't worry ..I doubt he would get custody just because you work 3 days a week ...

@Lovecats27 I'm sure you're already aware of this, but in cases of true court-ruled 50/50 there is no maintenance payable on either side x

@Lovecats27 try to be calm and not respond to comms from him immediately. Easier said than done I know. I went through this kind of thing for years. (An ex with money who used the family courts to punish me for leaving the abusive marriage.) You have an existing arrangement. You work and are able to care for him. You enable contact with the father. The status quo is more likely to be kept if DC is seeing both parents regularly and there is a regular pattern of contact. He is trying to bully and upset you. My ex also used the courts to financially punish me, though in the end I moved from a mediocre lawyer to a great one and ultimately it probably cost my ex thousands to simply get an hour extra contact on his birthday as extra!! Of course I spent lots of money in this process. What we had in place was reasonable and in the best interests of DC so it wasn't changed. The process was simply an attempt at bullying and attention grabbing. My suggestions: Record everything.
If you communicate with him, do it with half an eye that a court could see it so word it carefully to show you are keeping the best interests of the child as the priority and not reacting emotionally.
Be meticulous with filing the evidence.
Limit comms to one medium with him so he can't keep invading your life when he has no place to.
And if he persists, then get a good rather than mediocre family lawyer. At this point presumably a formal letter that says there is an established pattern of contact that is working well for the child should cover things. I hope it calms down.