hmm i thoughtt hat if the son's father is now properly on the tenancy and registered with the council as a joint tenant of that property -presumably along with you, then there shouldn't be a problem as he has a right to be there as a tenant whether he is your partner or not. You could probably arrange to sign the tenancy solely over to him if that's what you wanted
If he's not a proper tenant and you are the sole tenant then there could indeed be problems if you've left the property as the council could take your tenancy away and thereby make him homeless at which point he would need to get evicted and apply to the council for accommodation!