my ex has asked my 11 yr old son to text him regarding my new partner help ?

i split up with my husband 4 yrs ago and have been on my own with my 2 sons, my ex didnt want to know the kids until i have now met a new partner now he takes them out on a weekend, but when they come back he has questioned them and wants my 11yr old to text him when my new partner comes round or if i go out with him anywhere, i dont want to say anything to him as i dont want my oldest son getting into to trouble for telling me, im stuck please help ?