If he's named on the birth certificate as the father of your ds, he has parental responsibility and you cannnot legally stop him taking ds anywhere he wants.
But if, for example, he attempted to leave with ds while saying words to the effect that you won't see him again, you can call the police and ask them to intervene.
However, what's said in an argument doesn't necessarily come to pass and it's not unknown for twunts men to threaten they're not leaving without the dc or similar when, in fact, they've got no intention of saddling themselves with dc who'll cramp their style.
Locate your nearest Women's Aid office here: www.womensaid.org.uk give them a call, ask for recommendations for solicitors who specialise in family law, and make an urgent appointment to discuss your options as legal aid entitlement in family matters will cease after the end of this month and you could find yourself up the proverbial creek without a paddle if he makes good on his threat.
It may be that you have a vain hope your relationship will be rekindled or some such but, nevertheless, you're best advised to seek legal advice now as forewarned is forearmed and it very much sounds as if he's got, or is actively seeking, an ow to move in with.