What a dreadful mess, and what a repayment from your son for your kindness and support.
I am sorry if this offends you, but the logical action would be a termination followed by a vasectomy, and I say this as a means of protecting the unborn child; this feckless, irresponsible couple are totally unsuitable to be parents at present.
Have they made any plans for their future, or are they expecting you to continue housing and supporting them, whilst bearing the brunt of the child care?
If this appears harsh, it is because I regularly observe former pupils casually and badly fathering/ mothering, children, three in each case, without ever having worked but being supported by the state, (flats and houses provided in this expensive village) , whilst their hard-working contemporaries struggle to pay university loans and extortionate rents out of their earnings and postpone families until they can afford them.